Ligands for flt3 receptors

ABSTRACT

Ligands for flt3 receptors capable of transducing self-renewal signals to regulate the growth, proliferation or differentiation of progenitor cells and stem cells are disclosed. The invention is directed to flt3-L as an isolated protein, the DNA encoding the flt3-L, host cells transfected with cDNAs encoding flt3-L, compositions comprising flt3-L, methods of improving gene transfer to a mammal using flt3-L, and methods of improving transplantations using fit3-L. Flt3-L finds use in treating patients with anemia, AIDS and various cancers.

CROSS-REFERENCE TO RELATED APPLICATIONS

This Application is a Divisional of U.S. application Ser. No.08/444,626, filed May 19, 1995; which in turn is a Divisional of U.S.application Ser. No. 08/243,545, filed May 11, 1994 (now U.S. Pat. No.5,554,512); which in turn is a Continuation-In-Part of U.S. applicationSer. No. 08/209,502, filed Mar. 7, 1994 (now abandoned); which in turnis a Continuation-In-Part of U.S. application No. 08/162,407, filed Dec.3, 1993 (now abandoned); which in turn is a Continuation-In-Part of U.S.application No. 08/111,758, filed Aug. 25, 1993 (now abandoned); whichin turn is a Continuation-In-part of U.S. application No. 08/106,463,filed Aug. 12, 1993 (now abandoned); which in turn is aContinuation-In-part of U.S. application No. 08/068,394, filed May 24,1993 (now abandoned).

FIELD OF THE INVENTION

The present invention relates to mammalian flt3-ligands, the nucleicacids encoding such ligands, processes for production of recombinantflt3-ligands, pharmaceutical compositions containing such ligands, andtheir use in various therapies.

BACKGROUND OF THE INVENTION

Blood cells originate from hematopoietic stem cells that becomecommitted to differentiate along certain lineages, i.e., erythroid,megakaryocytic, granulocytic, monocytic, and lymphocytic. Cytokines thatstimulate the proliferation and maturation of cell precursors are calledcolony stimulating factors (“CSFs”). Several CSFs are produced byT-lymphocytes, including interleukin-3 (“IL-3”), granulocyte-monocyteCSF (GM-CSF), granulocyte CSF (G-CSF), and monocyte CSF (M-CSF). TheseCSFs affect both mature cells and stem cells. Heretofore no factors havebeen discovered that are able to predominantly affect stem cells.

Tyrosine kinase receptors (“TKRs”) are growth factor receptors thatregulate the proliferation and differentiation of a number of cells(Yarden, Y. & Ullrich, A. Annu. Rev. Biochem., 57, 443–478, 1988; andCadena, D. L. & Gill, G. N. FASEB J., 6, 2332–2337, 1992). Certain TKRsfunction within the hematopoietic system. For example, signaling throughthe colony-stimulating factor type 1 (“CSF-1”), receptor c-fms regulatesthe survival, growth and differentiation of monocytes (Stanley et al.,J. Cell Biochem., 21, 151–159, 1983). Steel factor (“SF”, also known asmast cell growth factor, stem cell factor or kit ligand), acting throughc-kit, stimulates the proliferation of cells in both myeloid andlymphoid compartments.

Flt3 (Rosnet et al. Oncogene, 6, 1641–1650, 1991) and flk-2 (Matthews etal., Cell, 65, 1143–1152, 1991) are variant forms of a TKR that isrelated to the c-fms and c-kit receptors. The flk-2 gene product isexpressed on hematopoietic and progenitor cells, while the flt3 geneproduct has a more general tissue distribution. The flt3 and flk-2receptor proteins are similar in amino acid sequence and vary at twoamino acid residues in the extracellular domain and diverge in a 31amino acid segment located near the C-termini (Lyman et al., Oncogene,8, 815–822, 1993).

Flt3-ligand (“flt3-L”) has been found to regulate the growth anddifferentiation of progenitor and stem cells and is likely to possessclinical utility in treating hematopoietic disorders, in particular,aplastic anemia and myelodysplastic syndromes. Additionally, flt3-L willbe useful in allogeneic, syngeneic or autologous bone marrow transplantsin patients undergoing cytoreductive therapies, as well as cellexpansion. Flt3-L will also be useful in gene therapy and progenitor andstem cell mobilization systems.

Cancer is treated with cytoreductive therapies that involveadministration of ionizing radiation or chemical toxins that killrapidly dividing cells. Side effects typically result from cytotoxiceffects upon normal cells and can limit the use of cytoreductivetherapies. A frequent side effect is myelosuppression, or damage to bonemarrow cells that give rise to white and red blood cells and platelets.As a result of myelosuppression, patients develop cytopenia, or bloodcell deficits, that increase risk of infection and bleeding disorders.

Cytopenias increase morbidity, mortality, and lead to under-dosing incancer treatment. Many clinical investigators have manipulatedcytoreductive therapy dosing regimens and schedules to increase dosingfor cancer therapy, while limiting damage to bone marrow. One approachinvolves bone marrow or peripheral blood cell transplants in which bonemarrow or circulating hematopoietic progenitor or stem cells are removedbefore cytoreductive therapy and then reinfused following therapy torestore hematopoietic function. U.S. Pat. No. 5,199,942, incorporatedherein by reference, describes a method for using GM-CSF, IL-3, SF,GM-CSF/IL-3 fusion proteins, erythropoietin (“EPO”) and combinationsthereof in autologous transplantation regimens.

High-dose chemotherapy is therapeutically beneficial because it canproduce an increased frequency of objective response in patients withmetastatic cancers, particularly breast cancer, when compared tostandard dose therapy. This can result in extended disease-freeremission for some even poor-prognosis patients. Nevertheless, high-dosechemotherapy is toxic and many resulting clinical complications arerelated to infections, bleeding disorders and other effects associatedwith prolonged periods of myelosuppression.

Myelodysplastic syndromes are stem cell disorders characterized byimpaired cellular maturation, progressive pancytopenia, and functionalabnormalities of mature cells. They have also been characterized byvariable degrees of cytopenia, ineffective erythropoiesis andmyelopoiesis with bone marrow cells that are normal or increased innumber and that have peculiar morphology. Bennett et. al. (Br. J.Haematol. 1982; 51:189–199) divided these disorders into five subtypes:refractory anemia, refractory anemia with ringed sideroblasts,refractory anemia with excess blasts, refractory anemia with excessblasts in transformation, and chronic myelomonocytic leukemia. Althougha significant percentage of these patients develop acute leukemia, amajority die from infectious or hemorrhagic complications. Treatment oftheses syndromes with retinoids, vitamin D, and cytarabine has not beensuccessful. Most of the patients suffering from these syndromes areelderly and are not suitable candidates for bone marrow transplantationor aggressive antileukemic chemotherapy.

Aplastic anemia is another disease entity that is characterized by bonemarrow failure and severe pancytopenia. Unlike myelodysplastic syndrome,the bone marrow is acellular or hypocellular in this disorder. Currenttreatments include bone marrow transplantation from a histocompatibledonor or immunosuppressive treatment with antithymocyte globulin (ATG).Similarly to myelodysplastic syndrome, most patients suffering from thissyndrome are elderly and are unsuitable for bone marrow transplantationor for aggressive antileukemic chemotherapy. Mortality in these patientsis exceedingly high from infectious or hemorrhagic complications.

Anemia is common in patients with acquired immune deficiency syndrome(AIDS). The anemia is usually more severe in patients receivingzidovudine therapy. Many important retroviral agents, anti-infectives,and anti-neoplastics suppress erythropoiesis. Recombinant EPO has beenshown to normalize the patient's hematocrit and hemaoglobin levels,however, usually very high doses are required. A growth factor thatstimulates proliferation of the erythroid lineage could be used alone orin combination with EPO or other growth factors to treat such patientsand reduce the number of transfusions required. A growth factor thatcould also increase the number of T cells would find particular use intreating AIDS patients.

SUMMARY OF THE INVENTION

The present invention pertains to biologically active flt3-ligand(flt3-L) as an isolated or homogeneous protein. In addition, theinvention is directed to isolated DNAs encoding flt3-L and to expressionvectors comprising a cDNA encoding flt3-L. Within the scope of thisinvention are host cells that have been transfected or transformed withexpression vectors that comprise a cDNA encoding flt3-L, and processesfor producing flt3-L by culturing such host cells under conditionsconducive to expression of flt3-L.

Flt3-L can be used to prepare pharmaceutical compositions to be used inallogeneic, syngeneic or autologous transplantation methods.Pharmaceutical compositions may comprise flt3-L alone or in combinationwith other growth factors, such as interleukins, colony stimulatingfactors, protein tyrosine kinases and cytokines.

The invention includes methods of using flt3-L compositions in genetherapy and in treatment of patients suffering from myelodysplasticsyndrome, aplastic anemia, HIV infection (AIDS) and cancers, such asbreast cancer, lymphoma, small cell lung cancer, multiple myeloma,neuroblastoma, acute leukemia, testicular tumors, and ovarian cancer.

The present invention also pertains to antibodies, and in particularmonoclonal antibodies, that are immunoreactive with flt3-L. Fusionproteins comprising a soluble portion of flt3-L and the constant domainof an immunoglobulin protein are also embodied in the invention.

The present invention also is directed to the use of flt3-L inperipheral blood progenitor or stem cell transplanation procedures.Typically, peripheral blood progenitor cells or stem cells are removedfrom a patient prior to myelosuppressive cytoreductive therapy, and thenreadministered to the patient concurrent with or following cytoreductivetherapy to counteract the myelosuppressive effects of such therapy. Thepresent invention provides for the use of an effective amount of flt3-Lin at least one of the following manners: (i) flt3-L is administered tothe patient prior to collection of the progenitor or stem cells toincrease or mobilize the numbers of such circulating cells; (ii)following collection of the patient's progenitor or stem cells, flt3-Lis used to expand such cells ex vivo; and (iii) flt3-L is administeredto the patient following transplantation of the collected progenitor orstem cells to facilitate engraftment thereof. The transplantation methodof the invention can further comprise the use of an effective amount ofa cytokine in sequential or concurrent combination with the flt3-L. Suchcytokines include, but are not limited to interleukins (“IL”) IL-1,IL-2, IL-3, IL-4, IL-5, IL-6, IL-7, IL-8, IL-9, IL-10, IL-11, IL-12,IL-13, IL-14 or IL-15, a CSF selected from the group consisting ofG-CSF, GM-CSF, M-CSF, or GM-CSF/IL-3 fusions, or other growth factorssuch as CSF-1, SF, EPO, leukemia inhibitory factor (“LIF”) or fibroblastgrowth factor (“FGF”). The flt3-L is also useful in the same way forsyngeneic or allogeneic transplantations.

The invention further includes a progenitor or stem cell expansion mediacomprising cell growth media, autologous serum, and flt3-L alone or incombination with a cytokine from the group listed above.

The invention further includes the use of flt3-L to expand progenitor orstem cells collected from umbilical cord blood. The expansion may beperformed with flt3-L alone or in sequential or concurrent combinationwith a cytokine from the group listed above.

The invention further includes the use of flt3-L in gene therapy. Flt3-Lpermits proliferation and culturing of the early hematopoieticprogenitor or stem cells that are to be transfected with exogenous DNAfor use in gene therapy. Alternatively, a cDNA encoding flt3-L may betransfected into cells in order to ultimately deliver its gene productto the targeted cell or tissue.

In addition, the invention includes the use of flt3-L to stimulateproduction of erythroid cells in vivo for the treatment of anemia. Suchuse comprises administering flt3-L to the patient in need of sucherythroid cell stimulation in conjunction with or followingcytoreductive therapy. The treatment can include co-administration ofanother growth factor selected from the cytokines from the group listedabove. Preferred cytokines for use in this treatment include EPO, IL-3,G-CSF and GM-CSF. Such treatment is especially useful for AIDS patients,and preferably for AIDS patients receiving AZT therapy.

Since flt3-L stimulates the production of stem cells, othernon-hematopoietic stem cells bearing flt3 receptors can be affected bythe flt3-L of the invention. Flt3-L is useful in in vitro fertilizationprocedures and can be used in vivo in the treatment of infertilityconditions. In the gut, the flt3 ligand is useful in treating intestinaldamage resulting from irradiation or chemotherapy. The flt3-L can bealso used to treat patients infected with the human immunodeficiencyvirus (HIV). Such treatment would encompass the administration of theflt3-L to stimulate in vivo production, as well as the ex vivoexpansion, of T cells and erythroid cells. Such treatment can preventthe deficiency of T cells, in particular CD4-positive T cells, and mayelevate the patient's immune reponse against the virus, therebyimproving the quality of life of the patient. The flt3-L can be used tostimulate the stem cells that lead to the development of hair follicles,thereby promoting hair growth.

In addition, flt3-L can be bound to a solid phase matrix and used toaffinity-purify or separate cells that express flt3 on their cellsurface. The invention encompasses separating cells having the flt3receptor on the surface thereof from a mixture of cells in solution,comprising contacting the cells in the mixture with a contacting surfacehaving a flt3-binding molecule thereon, and separating the contactingsurface and the solution. Once separated, the cells can be expanded exvivo using flt3-L and administered to a patient.

DETAILED DESCRIPTION OF THE INVENTION

A cDNA encoding murine flt3-L has been isolated and is disclosed in SEQID NO:1. A cDNA encoding human flt3-L also has been isolated and isdisclosed in SEQ ID NO:5. This discovery of cDNAs encoding murine andhuman flt3-L enables construction of expression vectors comprising cDNAsencoding flt3-L; host cells transfected or transformed with theexpression vectors; biologically active murine and human flt3-L ashomogeneous proteins; and antibodies immunoreactive with the murine andthe human flt3-L.

Flt3-L is useful in the enhancement, stimulation, proliferation orgrowth of cells expressing the flt3 receptor, includingnon-hematopoietic cells. Since the flt3 receptor is found in the brain,placenta, and tissues of nervous and hematopoietic origin, and findsdistribution in the testis, ovaries, lymph node, spleen, thymus andfetal liver, treatment of a variety of conditions associated with tissuedamage thereof is possible. While not limited to such, particular usesof the flt3-L are described infra.

As used herein, the term “flt3-L” refers to a genus of polypeptides thatbind and complex independently with flt3 receptor found on progenitorand stem cells. The term “flt3-L” encompasses proteins having the aminoacid sequence 1 to 231 of SEQ ID NO:2 or the amino acid sequence 1 to235 of SEQ ID NO:6, as well as those proteins having a high degree ofsimilarity or a high degree of identity with the amino acid sequence 1to 231 of SEQ ID NO:2 or the amino acid sequence 1 to 235 of SEQ IDNO:6, and which proteins are biologically active and bind the flt3receptor. In addition, the term refers to biologically active geneproducts of the DNA of SEQ ID NO:1 or SEQ ID NO:5. Further encompassedby the term “flt3-L” are the membrane-bound proteins (which include anintracellular region, a membrane region, and an extracellular region),and soluble or truncated proteins which comprise primarily theextracellular portion of the protein, retain biological activity and arecapable of being secreted. Specific examples of such soluble proteinsare those comprising the sequence of amino acids 28–163 of SEQ ID NO:2and amino acids 28–160 of SEQ ID NO:6.

The term “biologically active” as it refers to flt3-L, means that theflt3-L is capable of binding to flt3. Alternatively, “biologicallyactive” means the flt3-L is capable of transducing a stimulatory signalto the cell through the membrane-bound flt3.

“Isolated” means that flt3-L is free of association with other proteinsor polypeptides, for example, as a purification product of recombinanthost cell culture or as a purified extract.

A “flt3-L variant” as referred to herein, means a polypeptidesubstantially homologous to native flt3-L, but which has an amino acidsequence different from that of native flt3-L (human, murine or othermammalian species) because of one or more deletions, insertions orsubstitutions. The variant amino acid sequence preferably is at least80% identical to a native flt3-L amino acid sequence, most preferably atleast 90% identical. The percent identity may be determined, forexample, by comparing sequence information using the GAP computerprogram, version 6.0 described by Devereux et al. (Nucl. Acids Res.12:387, 1984) and available from the University of Wisconsin GeneticsComputer Group (UWGCG). The GAP program utilizes the alignment method ofNeedleman and Wunsch (J. Mol. Biol. 48:443, 1970), as revised by Smithand Waterman (Adv. Appl. Math 2:482, 1981). The preferred defaultparameters for the GAP program include: (1) a unary comparison matrix(containing a value of 1 for identities and 0 for non-identities) fornucleotides, and the weighted comparison matrix of Gribskov and Burgess,Nucl. Acids Res. 14:6745, 1986, as described by Schwartz and Dayhoff,eds., Atlas of Protein Sequence and Structure, National BiomedicalResearch Foundation, pp. 353–358, 1979; (2) a penalty of 3.0 for eachgap and an additional 0.10 penalty for each symbol in each gap; and (3)no penalty for end gaps. Variants may comprise conservativelysubstituted sequences, meaning that a given amino acid residue isreplaced by a residue having similar physiochemical characteristics.Examples of conservative substitutions include substitution of onealiphatic residue for another, such as Ile, Val, Leu, or Ala for oneanother, or substitutions of one polar residue for another, such asbetween Lys and Arg; Glu and Asp; or Gln and Asn. Other suchconservative substitutions, for example, substitutions of entire regionshaving similar hydrophobicity characteristics, are well known. Naturallyoccurring flt3-L variants are also encompassed by the invention.Examples of such variants are proteins that result from alternate mRNAsplicing events or from proteolytic cleavage of the flt3-L protein,wherein the flt3-L binding property is retained. Alternate splicing ofmRNA may yield a truncated but biologically active flt3-L protein, suchas a naturally occurring soluble form of the protein, for example.Variations attributable to proteolysis include, for example, differencesin the N- or C-termini upon expression in different types of host cells,due to proteolytic removal of one or more terminal amino acids from theflt3-L protein (generally from 1–5 terminal amino acids).

The term “autologous transplantation” is described in U.S. Pat. No.5,199,942, which is incorporated herein by reference. Briefly, the termmeans a method for conducting autologous hematopoietic progenitor orstem cell trans-plantation, comprising: (1) collecting hematopoieticprogenitor cells or stem cells from a patient prior to cytoreductivetherapy; (2) expanding the hematopoietic progenitor cells or stem cellsex vivo with flt3-L to provide a cellular preparation comprisingincreased numbers of hematopoietic progenitor cells or stem cells; and(3) administering the cellular preparation to the patient in conjunctionwith or following cytoreductive therapy. Progenitor and stem cells maybe obtained from peripheral blood harvest or bone marrow explants.Optionally, one or more cytokines, selected from the group listed abovecan be combined with flt3-L to aid in the proliferation of particularhematopoietic cell types or affect the cellular function of theresulting proliferated hematopoietic cell population. Of the foregoing,SF, IL-1, IL-3, EPO, G-CSF, GM-CSF and GM-CSF/IL-3 fusions arepreferred, with G-CSF, GM-CSF and GM-CSF/IL-3 fusions being especiallypreferred. The term “allogeneic transplantation” means a method in whichbone marrow or peripheral blood progenitor cells or stem cells areremoved from a mammal and administered to a different mammal of the samespecies. The term “syngeneic transplantation” means the bone marrowtransplantation between gentically identical mammals.

The transplantation method of the invention described above optionallycomprises a preliminary in vivo procedure comprising administeringflt3-L alone of in sequential or concurrent combination with arecruitment growth factor to a patient to recruit the hematopoieticcells into peripheral blood prior to their harvest. Suitable recruitmentfactors are listed above, and preferred recruitment factors are SF, IL-1and IL-3.

The method of the invention described above optionally comprises asubsequent in vivo procedure comprising administering flt3-L alone or insequential or concurrent combination with an engraftment growth factorto a patient following transplantation of the cellular preparation tofacilitate engraftment and augment proliferation of engraftedhematopoietic progenitor or stem cells from the cellular preparation.Suitable engraftment factors are listed above, and the preferredengraftment factors are GM-CSF, G-CSF, IL-3, IL-1, EPO and GM-CSF/IL-3fusions.

The invention further includes a progenitor or stem cell expansion mediacomprising cell growth media, autologous serum, and flt3-L alone or incombination with a cytokine growth factor from the list above. Preferredgrowth factors are SF, GM-CSF, IL-3, IL-1, G-CSF, EPO, and GM-CSF/IL-3fusions.

In particular, flt3-L can be used to stimulate the proliferation ofhematopoietic and non-hematopoietic stem cells. Such stimulation isbeneficial when specific tissue damage has occurred to these tissues. Assuch, flt3-L may be useful in treating neurological damage and may be agrowth factor for nerve cells. It is probable that flt3-L would beuseful in in vitro fertilization procedures and likely can be used invivo in the treatment of infertility conditions. Flt3-L would be usefulin treating intestinal damage resulting from irradiation orchemotherapy. Since the flt3 receptor is distributed on stem cellsleading to the development of hair follicles, flt3-L would likely beuseful to promote hair growth.

Since flt3-L has been shown to stimulate T cell proliferation as well aserythrocytes (see Examples, infra), flt3-L finds use in the treatment ofpatients infected with the human immunodeficiency virus (HIV). Suchtreatment would encompass the administration of flt3-L to stimulate invivo production, as well as the ex vivo expansion, of T cells. Inaddition, flt3-L can prevent the deficiency of CD4⁺ T cells. Suchtreatment may elevate or maintain a patient's immune reponse against thevirus, thereby improving or maintaining a patient's quality of life. Inaddition, such in vivo treatment would stimulate cells of the erythroidlineage, thereby improving a patient's hematocrit and hemaglobin levels.Flt3-L can be administered in this setting alone or in sequential orconcurrent combination with cytokines selected from the group listedabove.

Flt3-L is useful in gene therapy due to its specificity for progenitorand stem cells. Gene therapy involves administration of exogenousDNA-transfected cells to a host that are allowed to engraft. See e.g.,Boggs, International J. Cell Cloning, 8:80–96, (1990); Kohn et. al.,Cancer Invest., 7(2):179–192 (1989); Lehn, Bone Marrow Transpl.,5:287–293 (1990); and Verma, Scientific American, pp. 68–84 (1990).Using gene therapy methods known in the art, a method of transferring agene to a mammal comprises the steps of (a) culturing earlyhematopoietic cells in media comprising flt3-L alone or in sequential orconcurrent combination with a cyokine selected from the group listedabove; (b) transfecting the cultured cells from step (a) with theexogenous gene; and (c) administering the transfected cells to themammal. Within this method is the novel method of transfectingprogenitor or stem cells with a gene comprising the steps of: (a) and(b) above. Furthermore, using the same or simolar methods, the cDNAencoding the flt3-L can be transfected into such delivery cells todeliver the flt3-L gene product to the targetted tissue.

Example 1 describes the construction of a novel flt3:Fc fusion proteinutilized in the screening for flt3-L. Other antibody Fc regions may besubstituted for the human IgG1 Fc region described in Example 1. Othersuitable Fc regions are those that can bind with high affinity toprotein A or protein G, and include the Fc region of human IgG1 orfragments of the human or murine IgG1 Fc region, e.g., fragmentscomprising at least the hinge region so that interchain disulfide bondswill form. The flt3:Fc fusion protein offers the advantage of beingeasily purified. In addition, disulfide bonds form between the Fcregions of two separate fusion protein chains, creating dimers. Thedimeric flt3:Fc receptor was chosen for the potential advantage ofhigher affinity binding of flt3-L, in view of the possibility that theligand being sought would be multimeric.

As described supra., an aspect of the invention is soluble flt3-Lpolypeptides. Soluble flt3-L polypeptides comprise all or part of theextracellular domain of a native flt3-L but lack the transmembraneregion that would cause retention of the polypeptide on a cell membrane.Soluble flt3-L polypeptides advantageously comprise the native (or aheterologous) signal peptide when initially synthesized to promotesecretion, but the signal peptide is cleaved upon secretion of flt3-Lfrom the cell. Soluble flt3-L polypeptides encompassed by the inventionretain the ability to bind the flt3 receptor. Indeed, soluble flt3-L mayalso include part of the transmembrane region or part of the cytoplasmicdomain or other sequences, provided that the soluble flt3-L protein canbe secreted.

Soluble flt3-L may be identified (and distinguished from its non-solublemembrane-bound counterparts) by separating intact cells which expressthe desired protein from the culture medium, e.g., by centrifugation,and assaying the medium (supernatant) for the presence of the desiredprotein. The presence of flt3-L in the medium indicates that the proteinwas secreted from the cells and thus is a soluble form of the desiredprotein.

Soluble forms of flt3-L possess many advantages over the native boundflt3-L protein. Purification of the proteins from recombinant host cellsis feasible, since the soluble proteins are secreted from the cells.Further, soluble proteins are generally more suitable for intravenousadministration.

Examples of soluble flt3-L polypeptides include those comprising asubstantial portion of the extracellular domain of a native flt3-Lprotein. Such soluble mammalian flt3-L proteins comprise amino acids 28through 188 of SEQ ID NO:2 or amino acids 28 through 182 of SEQ ID NO:6.In addition, truncated soluble flt3-L proteins comprising less than theentire extracellular domain are included in the invention. Suchtruncated soluble proteins are represented by the sequence of aminoacids 28–163 of SEQ ID NO:2, and amino acids 28–160 of SEQ ID NO:6. Wheninitially expressed within a host cell, soluble flt3-L may additionallycomprise one of the heterologous signal peptides described below that isfunctional within the host cells employed. Alternatively, the proteinmay comprise the native signal peptide, such that the mammalian flt3-Lcomprises amino acids 1 through 188 of SEQ ID NO:2 or amino acids 1through 182 of SEQ ID NO:6. In one embodiment of the invention, solubleflt3-L was expressed as a fusion protein comprising (from N- toC-terminus) the yeast a factor signal peptide, a FLAG® peptide describedbelow and in U.S. Pat. No. 5,011,912, and soluble flt3-L consisting ofamino acids 28 to 188 of SEQ ID NO:2. This recombinant fusion protein isexpressed in and secreted from yeast cells. The FLAG® peptidefacilitates purification of the protein, and subsequently may be cleavedfrom the soluble flt3-L using bovine mucosal enterokinase. Isolated DNAsequences encoding soluble flt3-L proteins are encompassed by theinvention.

Truncated flt3-L, including soluble polypeptides, may be prepared by anyof a number of conventional techniques. A desired DNA sequence may bechemically synthesized using techniques known per se. DNA fragments alsomay be produced by restriction endonuclease digestion of a full lengthcloned DNA sequence, and isolated by electrophoresis on agarose gels.Linkers containing restriction endonuclease cleavage site(s) may beemployed to insert the desired DNA fragment into an expression vector,or the fragment may be digested at cleavage sites naturally presenttherein. The well known polymerase chain reaction procedure also may beemployed to amplify a DNA sequence encoding a desired protein fragment.As a further alternative, known mutagenesis techniques may be employedto insert a stop codon at a desired point, e.g., immediately downstreamof the codon for the last amino acid of the extracellular domain.

In another approach, enzymatic treatment (e.g., using Bal 31exonuclease) may be employed to delete terminal nucleotides from a DNAfragment to obtain a fragment having a particular desired terminus.Among the commercially available linkers are those that can be ligatedto the blunt ends produced by Bal 31 digestion, and which containrestriction endonuclease cleavage site(s). Alternatively,oligonucleotides that reconstruct the N- or C-terminus of a DNA fragmentto a desired point may be synthesized and -ligated to the DNA fragment.The synthesized oligonucleotide may contain a restriction endonucleasecleavage site upstream of the desired coding sequence and position aninitiation codon (ATG) at the N-terminus of the coding sequence.

As stated above, the invention provides isolated or homogeneous flt3-Lpolypeptides, both recombinant and non-recombinant. Variants andderivatives of native flt3-L proteins that retain the desired biologicalactivity (e.g., the ability to bind flt3) may be obtained by mutationsof nucleotide sequences coding for native flt3-L polypeptides.Alterations of the native amino acid sequence may be accomplished by anyof a number of conventional methods. Mutations can be introduced atparticular loci by synthesizing oligonucleotides containing a mutantsequence, flanked by restriction sites enabling ligation to fragments ofthe native sequence. Following ligation, the resulting reconstructedsequence encodes an analog having the desired amino acid insertion,substitution, or deletion.

Alternatively, oligonucleotide-directed site-specific mutagenesisprocedures can be employed to provide an altered gene whereinpredetermined codons can be altered by substitution, deletion orinsertion. Exemplary methods of making the alterations set forth aboveare disclosed by Walder et al. (Gene 42:133, 1986); Bauer et al. (Gene37:73, 1985); Craik (BioTechniques, January 1985, 12–19); Smith et al.(Genetic Engineering: Principles and Methods, Plenum Press, 1981);Kunkel (Proc. Natl. Acad. Sci. USA 82:488, 1985); Kunkel et al. (Methodsin Enzymol. 154:367, 1987); and U.S. Pat. Nos. 4,518,584 and 4,737,462all of which are incorporated by reference.

Flt3-L may be modified to create flt3-L derivatives by forming covalentor aggregative conjugates with other chemical moieties, such as glycosylgroups, lipids, phosphate, acetyl groups and the like. Covalentderivatives of flt3-L may be prepared by linking the chemical moietiesto functional groups on flt3-L amino acid side chains or at theN-terminus or C-terminus of a flt3-L polypeptide or the extracellulardomain thereof. Other derivatives of flt3-L within the scope of thisinvention include covalent or aggregative conjugates of flt3-L or itsfragments with other proteins or polypeptides, such as by synthesis inrecombinant culture as N-terminal or C-terminal fusions. For example,the conjugate may comprise a signal or leader polypeptide sequence (e.g.the α-factor leader of Saccharomyces) at the N-terminus of a flt3-Lpolypeptide. The signal or leader peptide co-translationally orpost-translationally directs transfer of the conjugate from its site ofsynthesis to a site inside or outside of the cell membrane or cell wall.

Flt3-L polypeptide fusions can comprise peptides added to facilitatepurification and identification of flt3-L. Such peptides include, forexample, poly-His or the antigenic identification peptides described inU.S. Pat. No. 5,011,912 and in Hopp et al., Bio/Technology 6:1204, 1988.

The invention further includes flt3-L polypeptides with or withoutassociated native-pattern glycosylation. Flt3-L expressed in yeast ormammalian expression systems (e.g., COS-7 cells) may be similar to orsignificantly different from a native flt3-L polypeptide in molecularweight and glycosylation pattern, depending upon the choice ofexpression system. Expression of flt3-L polypeptides in bacterialexpression systems, such as E. coli, provides non-glycosylatedmolecules.

Equivalent DNA constructs that encode various additions or substitutionsof amino acid residues or sequences, or deletions of terminal orinternal residues or sequences not needed for biological activity orbinding are encompassed by the invention. For example, N-glycosylationsites in the flt3-L extracellular domain can be modified to precludeglycosylation, allowing expression of a reduced carbohydrate analog inmammalian and yeast expression systems. N-glycosylation sites ineukaryotic polypeptides are characterized by an amino acid tripletAsn-X-Y, wherein X is any amino acid except Pro and Y is Ser or Thr. Themurine and human flt3-L proteins each comprise two such triplets, atamino acids 127–129 and 152–154 of SEQ ID NO:2, and at amino acids126–128 and 150–152 of SEQ ID NO:6, respectively. Appropriatesubstitutions, additions or deletions to the nucleotide sequenceencoding these triplets will result in prevention of attachment ofcarbohydrate residues at the Asn side chain. Alteration of a singlenucleotide, chosen so that Asn is replaced by a different amino acid,for example, is sufficient to inactivate an N-glycosylation site. Knownprocedures for inactivating N-glycosylation sites in proteins includethose described in U.S. Pat. No. 5,071,972 and EP 276,846, herebyincorporated by reference.

In another example, sequences encoding Cys residues that are notessential for biological activity can be altered to cause the Cysresidues to be deleted or replaced with other amino acids, preventingformation of incorrect intramolecular disulfide bridges uponrenaturation. Other equivalents are prepared by modification of adjacentdibasic amino acid residues to enhance expression in yeast systems inwhich KEX2 protease activity is present. EP 212,914 discloses the use ofsite-specific mutagenesis to inactivate KEX2 protease processing sitesin a protein. KEX2 protease processing sites are inactivated bydeleting, adding or substituting residues to alter Arg-Arg, Arg-Lys, andLys-Arg pairs to eliminate the occurrence of these adjacent basicresidues. Lys-Lys pairings are considerably less susceptible to KEX2cleavage, and conversion of Arg-Lys or Lys-Arg to Lys-Lys represents aconservative and preferred approach to inactivating KEX2 sites. Bothmurine and human flt3-L contain two KEX2 protease processing sites atamino acids 216–217 and 217–218 of SEQ ID NO:2 and at amino acids211–212 and 212–213 of SEQ ID NO:6, respectively.

Nucleic acid sequences within the scope of the invention includeisolated DNA and RNA sequences that hybridize to the native flt3-Lnucleotide sequences disclosed herein under conditions of moderate orsevere stringency, and which encode biologically active flt3-L.Conditions of moderate stringency, as defined by Sambrook et al.Molecular Cloning: A Laboratory Manual, 2 ed. Vol. 1, pp. 1.101–104,Cold Spring Harbor Laboratory Press, (1989), include use of a prewashingsolution of 5×SSC, 0.5% SDS, 1.0 mM EDTA (pH 8.0) and hybridizationconditions of about 55° C., 5×SSC, overnight. Conditions of severestringency include higher temperatures of hybridization and washing. Theskilled artisan will recognize that the temperature and wash solutionsalt concentration may be adjusted as necessary according to factorssuch as the length of the probe.

Due to the known degeneracy of the genetic code wherein more than onecodon can encode the same amino acid, a DNA sequence may vary from thatshown in SEQ ID NO:1 and SEQ ID NO:5 and still encode an flt3-L proteinhaving the amino acid sequence of SEQ ID NO:2 and SEQ ID NO:6,respectively. Such variant DNA sequences may result from silentmutations (e.g., occurring during PCR amplification), or may be theproduct of deliberate mutagenesis of a native sequence.

The invention provides equivalent isolated DNA sequences encodingbiologically active flt3-L, selected from: (a) DNA derived from thecoding region of a native mammalian flt3-L gene; (b) cDNA comprising thenucleotide sequence presented in SEQ ID NO:1 or SEQ ID NO:5; (c) DNAcapable of hybridization to a DNA of (a) under moderately stringentconditions and which encodes biologically active flt3-L; and (d) DNAwhich is degenerate as a result of the genetic code to a DNA defined in(a), (b) or (c) and which encodes biologically active flt3-L. Flt3-Lproteins encoded by such DNA equivalent sequences are encompassed by theinvention.

DNA that are equivalents to the DNA sequence of SEQ ID NO:1 or SEQ IDNO:5, will hybridize under moderately stringent conditions to the nativeDNA sequence that encode polypeptides comprising amino acid sequences of28–163 of SEQ ID NO:2 or 28–160 of SEQ ID NO:6. Examples of flt3-Lproteins encoded by such DNA, include, but are not limited to, fit3-Lfragments (soluble or membrane-bound) and flt3-L proteins comprisinginactivated N-glycosylation site(s), inactivated KEX2 proteaseprocessing site(s), or conservative amino acid substitution(s), asdescribed above. Flt3-L proteins encoded by DNA derived from othermammalian species, wherein the DNA will hybridize to the cDNA of SEQ IDNO:1 or SEQ ID NO:5, are also encompassed.

Variants possessing the requisite ability to bind flt3 receptor may beidentified by any suitable assay. Biological activity of flt3-L may bedetermined, for example, by competition for binding to the ligandbinding domain of flt3 receptor (i.e. competitive binding assays).

One type of a competitive binding assay for a flt3-L polypeptide uses aradiolabeled, soluble human flt3-L and intact cells expressing cellsurface flt3 receptors. Instead of intact cells, one could substitutesoluble flt3 receptors (such as a flt3:Fc fusion protein) bound to asolid phase through the interaction of a Protein A, Protein G or anantibody to the flt3 or Fc portions of the molecule, with the Fc regionof the fusion protein. Another type of competitive binding assayutilizes radiolabeled soluble flt3 receptors such as a flt3:Fc fusionprotein, and intact cells expressing flt3-L. Alternatively, solubleflt3-L could be bound to a solid phase to positively select flt3expressing cells.

Competitive binding assays can be performed following conventionalmethodology. For example, radiolabeled flt3-L can be used to competewith a putative flt3-L homolog to assay for binding activity againstsurface-bound flt3 receptors. Qualitative results can be obtained bycompetitive autoradiographic plate binding assays, or Scatchard plotsmay be utilized to generate quantitative results.

Alternatively, flt3-binding proteins, such as flt3-L and anti-flt3antibodies, can be bound to a solid phase such as a columnchromatography matrix or a similar substrate suitable for identifying,separating or purifying cells that express the flt3 receptor on theirsurface. Binding of flt3-binding proteins to a solid phase contactingsurface can be accomplished by any means, for example, by constructing aflt3-L:Fc fusion protein and binding such to the solid phase through theinteraction of Protein A or Protein G. Various other means for fixingproteins to a solid phase are well known in the art and are suitable foruse in the present invention. For example, magnetic microspheres can becoated with flt3-binding proteins and held in the incubation vesselthrough a magnetic field. Suspensions of cell mixtures containinghematopoietic progenitor or stem cells are contacted with the solidphase that has flt3-binding proteins thereon. Cells having the flt3receptor on their surface bind to the fixed flt3-binding protein andunbound cells then are washed away. This affinity-binding method isuseful for purifying, screening or separating such flt3-expressing cellsfrom solution. Methods of releasing positively selected cells from thesolid phase are known in the art and encompass, for example, the use ofenzymes. Such enzymes are preferably non-toxic and non-injurious to thecells and are preferably directed to cleaving the cell-surface bindingpartner. In the case of flt3:flt3-L interactions, the enzyme preferablywould cleave the flt3 receptor, thereby freeing the resulting cellsuspension from the “foreign” flt3-L material. The purified cellpopulation then may be expanded ex vivo prior to transplantation to apatient in an amount sufficient to reconstitute the patient'shematopoietic and immune system.

Alternatively, mixtures of cells suspected of containing flt3⁺ cellsfirst can be incubated with a biotinylated flt3-binding protein.Incubation periods are typically at least one hour in duration to ensuresufficient binding to flt3. The resulting mixture then is passed througha column packed with avidin-coated beads, whereby the high affinity ofbiotin for avidin provides the binding of the cell to the beads. Use ofavidin-coated beads is known in the art. See Berenson, et al. J. Cell.Biochem., 10D:239 (1986). Wash of unbound material and the release ofthe bound cells is performed using conventional methods.

In the methods described above, suitable flt3-binding proteins areflt3-L, anti-flt3 antibodies, and other proteins that are capable ofhigh-affinity binding of flt3. A preferred flt3-binding protein isflt3-L.

As described above, flt3-L of the invention can be used to separatecells expressing flt3 receptors. In an alternative method, fit3-L or anextracellular domain or a fragrnent thereof can be conjugated to adetectable moiety such as ¹²⁵I to detect flt3 expressing cells.Radiolabeling with ¹²⁵I can be performed by any of several standardmethodologies that yield a functional ¹²⁵I-flt3-L molecule labeled tohigh specific activity. Or an iodinated or biotinylated antibody againstthe flt3 region or the Fc region of the molecule could be used. Anotherdetectable moiety such as an enzyme that can catalyze a colorimetric orfluorometric reaction, biotin or avidin may be used. Cells to be testedfor flt3 receptor expression can be contacted with labeled flt3-L. Afterincubation, unbound labeled flt3-L is removed and binding is measuredusing the detectable moiety.

The binding characteristics of flt3-L (including variants) may also bedetermined using the conjugated, soluble flt3 receptors (for example,¹²⁵I-flt3:Fc) in competition assays similar to those described above. Inthis case, however, intact cells expressing flt3 receptors, or solubleflt3 receptors bound to a solid substrate, are used to measure theextent to which a sample containing a putative flt3-L variant competesfor binding with a conjugated a soluble flt3 to flt3-L.

Other means of assaying for flt3-L include the use of anti-flt3-Lantibodies, cell lines that proliferate in response to flt3-L, orrecombinant cell lines that express flt3 receptor and proliferate in thepresenvce of flt3-L. For example, the BAF/BO3 cell line lacks the flt3receptor and is IL-3 dependent. (See Hatakeyama, et al., Cell, 59:837–845 (1989)). BAF/BO3 cells transfected with an expression vectorcomprising the flt3 receptor gene proliferate in response to either IL-3or flt3-L. An example of a suitable expression vector for transfectionof flt3 is the pCAV/NOT plasmid, see Mosley et al., Cell, 59: 335–348(1989).

Flt3-L polypeptides may exist as oligomers, such as covalently-linked ornon-convalently-linked dimers or trimers. Oligomers may be linked bydisulfide bonds formed between cysteine residues on different flt3-Lpolypeptides. In one embodiment of the invention, a flt3-L dimer iscreated by fusing flt3-L to the Fc region of an antibody (e.g., IgG1) ina manner that does not interfere with binding of flt3-L to theflt3-ligand-binding domain. The Fc polypeptide preferably is fused tothe C-terminus of a soluble flt3-L (comprising only the extracellulardomain). General preparation of fusion proteins comprising heterologouspolypeptides fused to various portions of antibody-derived polypeptides(including the Fc domain) has been described, e.g., by Ashkenazi et al.(PNAS USA 88:10535, 1991) and Byrn et al. (Nature 344:677, 1990), herebyincorporated by reference. A gene fusion encoding the flt3-L:Fc fusionprotein is inserted into an appropriate expression vector. Flt3-L:Fcfusion proteins are allowed to assemble much like antibody molecules,whereupon interchain disulfide bonds form between Fc polypeptides,yielding divalent flt3-L. If fusion proteins are made with both heavyand light chains of an antibody, it is possible to form a flt3-Loligomer with as many as four flt3-L extracellular regions.Alternatively, one can link two soluble flt3-L domains with a peptidelinker.

Recombinant expression vectors containing a DNA encoding flt3-L can beprepared using well known methods. The expression vectors include aflt3-L DNA sequence operably linked to suitable transcriptional ortranslational regulatory nucleotide sequences, such as those derivedfrom a mammalian, microbial, viral, or insect gene. Examples ofregulatory sequences include transcriptional promoters, operators, orenhancers, an mRNA ribosomal binding site, and appropriate sequenceswhich control transcription and translation initiation and termination.Nucleotide sequences are “operably linked” when the regulatory sequencefunctionally relates to the flt3-L DNA sequence. Thus, a promoternucleotide sequence is operably linked to a flt3-L DNA sequence if thepromoter nucleotide sequence controls the transcription of the flt3-LDNA sequence. The ability to replicate in the desired host cells,usually conferred by an origin of replication, and a selection gene bywhich transformants are identified, may additionally be incorporatedinto the expression vector.

In addition, sequences encoding appropriate signal peptides that are notnaturally associated with flt3-L can be incorporated into expressionvectors. For example, a DNA sequence for a signal peptide (secretoryleader) may be fused in-frame to the flt3-L sequence so that flt3-L isinitially translated as a fusion protein comprising the signal peptide.A signal peptide that is functional in the intended host cells enhancesextracellular secretion of the flt3-L polypeptide. The signal peptidemay be cleaved from the flt3-L polypeptide upon secretion of flt3-L fromthe cell.

Suitable host cells for expression of flt3-L polypeptides includeprokaryotes, yeast or higher eukaryotic cells. Appropriate cloning andexpression vectors for use with bacterial, fungal, yeast, and mammaliancellular hosts are described, for example, in Pouwels et al. CloningVectors: A Laboratory Manual, Elsevier, N.Y., (1985). Cell-freetranslation systems could also be employed to produce flt3-Lpolypeptides using RNAs derived from DNA constructs disclosed herein.

Prokaryotes include gram negative or gram positive organisms, forexample, E. coli or Bacilli. Suitable prokaryotic host cells fortransformation include, for example, E. coli, Bacillus subtilis,Salmonella typhimurium, and various other species within the generaPseudomonas, Streptomyces, and Staphylococcus. In a prokaryotic hostcell, such as E. coli, a flt3-L polypeptide may include an N-terminalmethionine residue to facilitate expression of the recombinantpolypeptide in the prokaryotic host cell. The N-terminal Met may becleaved from the expressed recombinant flt3-L polypeptide.

Expression vectors for use in prokaryotic host cells generally compriseone or more phenotypic selectable marker genes. A phenotypic selectablemarker gene is, for example, a gene encoding a protein that confersantibiotic resistance or that supplies an autotrophic requirement.Examples of useful expression vectors for prokaryotic host cells includethose derived from commercially available plasmids such as the cloningvector pBR322 (ATCC 37017). pBR322 contains genes for ampicillin andtetracycline resistance and thus provides simple means for identifyingtransformed cells. To construct en expression vector using pBR322, anappropriate promoter and a flt3-L DNA sequence are inserted into thepBR322 vector. Other commercially available vectors include, forexample, pKK223–3 (Pharmacia Fine Chemicals, Uppsala, Sweden) and pGEM1(Promega Biotec, Madison, Wis., USA).

Promoter sequences commonly used for recombinant prokaryotic host cellexpression vectors include β-lactamase (penicillinase), lactose promotersystem (Chang et al., Nature 275:615, 1978; and Goeddel et al., Nature281:544, 1979), tryptophan (trp) promoter system (Goeddel et al., Nucl.Acids Res. 8:4057, 1980; and EP-A-36776) and tac promoter (Maniatis,Molecular Cloning: A Laboratory Manual, Cold Spring Harbor Laboratory,p. 412, 1982). A particularly useful prokaryotic host cell expressionsystem employs a phage λP_(L) promoter and a cI857ts thermolabilerepressor sequence. Plasmid vectors available from the American TypeCulture Collection which incorporate derivatives of the λP_(L) promoterinclude plasmid pHUB2 (resident in E. coli strain JMB9 (ATCC 37092)) andpPLc28 (resident in E. coli RR1 (ATCC 53082)).

Flt3-L polypeptides alternatively may be expressed in yeast host cells,preferably from the Saccharomyces genus (e.g., S. cerevisiae). Othergenera of yeast, such as Pichia, K. lactis or Kluyveromyces, may also beemployed. Yeast vectors will often contain an origin of replicationsequence from a 2μ yeast plasmid, an autonomously replicating sequence(ARS), a promoter region, sequences for polyadenylation, sequences fortranscription termination, and a selectable marker gene. Suitablepromoter sequences for yeast vectors include, among others, promotersfor metallothionein, 3-phosphoglycerate kinase (Hitzeman et al., J.Biol. Chem. 255:2073, 1980) or other glycolytic enzymes (Hess et al., J.Adv. Enzyme Reg. 7:149, 1968; and Holland et al., Biochem. 17:4900,1978), such as enolase, glyceraldehyde-3-phosphate dehydrogenase,hexokinase, pyruvate decarboxylase, phosphofructokinase,glucose-6-phosphate isomerase, 3-phosphoglycerate mutase, pyruvatekinase, triosephosphate isomerase, phosphoglucose isomerase, andglucokinase. Other suitable vectors and promoters for use in yeastexpression are further described in Hitzeman, EPA-73,657 or in Fleer et.al., Gene, 107:285–195 (1991); and van den Berg et. al., Bio/Technology,8:135–139 (1990). Another alternative is the glucose-repressible ADH2promoter described by Russell et al. (J. Biol. Chem. 258:2674, 1982) andBeier et al. (Nature 300:724, 1982). Shuttle vectors replicable in bothyeast and E. coli may be constructed by inserting DNA sequences frompBR322 for selection and replication in E. coli (Amp^(r) gene and originof replication) into the above-described yeast vectors.

The yeast α-factor leader sequence may be employed to direct secretionof the flt3-L polypeptide. The α-factor leader sequence is ofteninserted between the promoter sequence and the structural gene sequence.See, e.g., Kurjan et al., Cell 30:933, 1982; Bitter et al., Proc. Natl.Acad. Sci. USA 81:5330, 1984; U.S. Pat. No. 4,546,082; and EP 324,274.Other leader sequences suitable for facilitating secretion ofrecombinant polypeptides from yeast hosts are known to those of skill inthe art. A leader sequence may be modified near its 3′ end to containone or more restriction sites. This will facilitate fusion of the leadersequence to the structural gene.

Yeast transformation protocols are known to those of skill in the art.One such protocol is described by Hinnen et al., Proc. Natl. Acad. Sci.USA 75:1929, 1978. The Hinnen et al. protocol selects for Trp⁺transformants in a selective medium, wherein the selective mediumconsists of 0.67% yeast nitrogen base, 0.5% casamino acids, 2% glucose,10 μg /ml adenine and 20 μg/ml uracil.

Yeast host cells transformed by vectors containing ADH2 promotersequence may be grown for inducing expression in a “rich” medium. Anexample of a rich medium is one consisting of 1% yeast extract, 2%peptone, and 1% glucose supplemented with 80 μg/ml adenine and 80 μg/mluracil. Derepression of the ADH2 promoter occurs when glucose isexhausted from the medium.

Mammalian or insect host cell culture systems could also be employed toexpress recombinant flt3-L polypeptides. Baculovirus systems forproduction of heterologous proteins in insect cells are reviewed byLuckow and Summers, Bio/Technology 6:47 (1988). Established cell linesof mammalian origin also may be employed. Examples of suitable mammalianhost cell lines include the COS-7 line of monkey kidney cells (ATCC CRL1651) (Gluzman et al., Cell 23:175, 1981), L cells, C127 cells, 3T3cells (ATCC CCL 163), Chinese hamster ovary (CHO) cells, HeLa cells, andBHK (ATCC CRL 10) cell lines, and the CV-1/EBNA-1 cell line derived fromthe African green monkey kidney cell line CVI (ATCC CCL 70) as describedby McMahan et al. (EMBO J. 10: 2821, 1991).

Transcriptional and translational control sequences for mammalian hostcell expression vectors may be excised from viral genomes. Commonly usedpromoter sequences and enhancer sequences are derived from Polyomavirus, Adenovirus 2, Simian Virus 40 (SV40), and human cytomegalovirus.DNA sequences derived from the SV40 viral genome, for example, SV40origin, early and late promoter, enhancer, splice, and polyadenylationsites may be used to provide other genetic elements for expression of astructural gene sequence in a mammalian host cell. Viral early and latepromoters are particularly useful because both are easily obtained froma viral genome as a fragment which may also contain a viral origin ofreplication (Fiers et al., Nature 273:113, 1978). Smaller or larger SV40fragments may also be used, provided the approximately 250 bp sequenceextending from the Hind III site toward the Bgl I site located in theSV40 viral origin of replication site is included.

Exemplary expression vectors for use in mammalian host cells can beconstructed as disclosed by Okayama and Berg (Mol. Cell. Biol. 3:280,1983). A useful system for stable high level expression of mammaliancDNAs in C127 murine mammary epithelial cells can be constructedsubstantially as described by Cosman et al. (Mol. Immunol. 23:935,1986). A useful high expression vector, PMLSV N1/N4, described by Cosmanet al., Nature 312:768, 1984 has been deposited as ATCC 39890.Additional useful mammalian expression vectors are described inEP-A-0367566, and in U.S. patent application Ser. No. 07/701,415, filedMay 16, 1991, incorporated by reference herein. The vectors may bederived from retroviruses. In place of the native signal sequence, aheterologous signal sequence may be added, such as the signal sequencefor IL-7 described in U.S. Pat. No. 4,965,195; the signal sequence forIL-2 receptor described in Cosman et al.,Nature 312:768 (1984); the IL-4signal peptide described in EP 367,566; the type I IL-1 receptor signalpeptide described in U.S. Pat. No. 4,968,607; and the type II IL-1receptor signal peptide described in EP 460,846.

Flt3-L as an isolated or homogeneous protein according to the inventionmay be produced by recombinant expression systems as described above orpurified from naturally occurring cells. Flt3-L can be purified tosubstantial homogeneity, as indicated by a single protein band uponanalysis by SDS-polyacrylamide gel electrophoresis (SDS-PAGE).

One process for producing flt3-L comprises culturing a host celltransformed with an expression vector comprising a DNA sequence thatencodes flt3-L under conditions sufficient to promote expression offlt3-L. Flt3-L is then recovered from culture medium or cell extracts,depending upon the expression system employed. As is known to theskilled artisan, procedures for purifying a recombinant protein willvary according to such factors as the type of host cells employed andwhether or not the recombinant protein is secreted into the culturemedium.

For example, when expression systems that secrete the recombinantprotein are employed, the culture medium first may be concentrated usinga commercially available protein concentration filter, for example, anAmicon or Millipore Pellicon ultrafiltration unit. Following theconcentration step, the concentrate can be applied to a purificationmatrix such as a gel filtration medium. Alternatively, an anion exchangeresin can be employed, for example, a matrix or substrate having pendantdiethylaminoethyl (DEAE) groups. The matrices can be acrylamide,agarose, dextran, cellulose or other types commonly employed in proteinpurification. Alternatively, a cation exchange step can be employed.Suitable cation exchangers include various insoluble matrices comprisingsulfopropyl or carboxymethyl groups. Sulfopropyl groups are preferred.Finally, one or more reversed-phase high performance liquidchromatography (RH-HPLC) steps employing hydrophobic RP-HPLC media,(e.g., silica gel having pendant methyl or other aliphatic groups) canbe employed to further purify flt3-L. Some or all of the foregoingpurification steps, in various combinations, are well known and can beemployed to provide a substantially homogeneous recombinant protein.

It is possible to utilize an affinity column comprising the ligandbinding domain of flt3 receptors to affinity-purify expressed flt3-Lpolypeptides. Flt3-L polypeptides can be removed from an affinity columnusing conventional techniques, e.g., in a high salt elution buffer andthen dialyzed into a lower salt buffer for use or by changing pH orother components depending on the affinity matrix utilized.Alternatively, the affinity column may comprise an antibody that bindsflt3-L. Example 6 describes a procedure for employing flt3-L of theinvention to generate monoclonal antibodies directed against flt3-L.

Recombinant protein produced in bacterial culture is usually isolated byinitial disruption of the host cells, centrifugation, extraction fromcell pellets if an insoluble polypeptide, or from the supernatant fluidif a soluble polypeptide, followed by one or more concentration,salting-out, ion exchange, affinity purification or size exclusionchromatography steps. Finally, RP-HPLC can be employed for finalpurification steps. Microbial cells can be disrupted by any convenientmethod, including freeze-thaw cycling, sonication, mechanicaldisruption, or use of cell lysing agents.

Transformed yeast host cells are preferably employed to express flt3-Las a secreted polypeptide in order to simplify purification. Secretedrecombinant polypeptide from a yeast host cell fermentation can bepurified by methods analogous to those disclosed by Urdal et al. (J.Chromatog. 296:171, 1984). Urdal et al. describe two sequential,reversed-phase HPLC steps for purification of recombinant human IL-2 ona preparative HPLC column.

Antisense or sense oligonucleotides comprising a single-stranded nucleicacid sequence (either RNA or DNA) capable of binding to a target flt3-LMRNA sequence (forming a duplex) or to the flt3-L sequence in thedouble-stranded DNA helix (forming a triple helix) can be made accordingto the invention. Antisense or sense oligonucleotides, according to thepresent invention, comprise a fragment of the coding region of flt3-LcDNA. Such a fragment generally comprises at least about 14 nucleotides,preferably from about 14 to about 30 nucleotides. The ability to createan antisense or a sense oligonucleotide, based upon a cDNA sequence fora given protein is described in, for example, Stein and Cohen, CancerRes. 48:2659, 1988 and van der Krol et al., BioTechniques 6:958, 1988.

Binding of antisense or sense oligonucleotides to target nucleic acidsequences results in the formation of complexes that block translation(RNA) or transcription (DNA) by one of several means, including enhanceddegradation of the duplexes, premature termination of transcription ortranslation, or by other means. The antisense oligonucleotides thus maybe used to block expression of flt3-L proteins. Antisense or senseoligonucleotides further comprise oligonucleotides having modifiedsugar-phosphodiester backbones (or other sugar linkages, such as thosedescribed in WO91/06629) and wherein such sugar linkages are resistantto endogenous nucleases. Such oligonucleotides with resistant sugarlinkages are stable in vivo (i.e., capable of resisting enzymaticdegradation) but retain sequence specificity to be able to bind totarget nucleotide sequences. Other examples of sense or antisenseoligonucleotides include those oligonucleotides which are covalentlylinked to organic moieties, such as those described in WO 90/10448, andother moieties that increases affinity of the oligonucleotide for atarget nucleic acid sequence, such as poly-(L-lysine). Further still,intercalating agents, such as ellipticine, and alkylating agents ormetal complexes may be attached to sense or antisense oligonucleotidesto modify binding specificities of the antisense or senseoliginucleotide for the target nucleotide sequence.

Antisense or sense oligonucleotides may be introduced into a cellcontaining the target nucleic acid sequence by any gene transfer method,including, for example, CaP₄-mediated DNA transfection, electroporation,or by using gene transfer vectors such as Epstein-Barr virus. Antisenseor sense oligonucleotides are preferably introduced into a cellcontaining the target nucleic acid sequence by insertion of theantisense or sense oligonucleotide into a suitable retroviral vector,then contacting the cell with the retrovirus vector containing theinserted sequence, either in vivo or ex vivo. Suitable retroviralvectors include, but are not limited to, the murine retrovirus M-MuLV,N2 (a retrovirus derived from M-MuLV), or or the double copy vectorsdesignated DCT5A, DCT5B and DCT5C (see PCT Application U.S. 90/02656).

Sense or antisense oligonucleotides also may be introduced into a cellcontaining the target nucleotide sequence by formation of a conjugatewith a ligand binding molecule, as described in WO 91/04753. Suitableligand binding molecules include, but are not limited to, cell surfacereceptors, growth factors, other cytokines, or other ligands that bindto cell surface receptors. Preferably, conjugation of the ligand bindingmolecule does not substantially interfere with the ability of the ligandbinding molecule to bind to its corresponding molecule or receptor, orblock entry of the sense or antisense oligonucleotide or its conjugatedversion into the cell.

Alternatively, a sense or an antisense oligonucleotide may be introducedinto a cell containing the target nucleic acid sequence by formation ofan oligonucleotide-lipid complex, as described in WO 90/10448. The senseor antisense oligonucleotide-lipid complex is preferably dissociatedwithin the cell by an endogenous lipase.

Flt3-L polypeptides of the invention can be formulated according toknown methods used to prepare pharmaceutically useful compositions.Flt3-L can be combined in admixture, either as the sole active materialor with other known active materials, with pharmaceutically suitablediluents (e.g., Tris-HCl, acetate, phosphate), preservatives (e.g.,Thimerosal, benzyl alcohol, parabens), emulsifiers, solubilizers,adjuvants and/or carriers. Suitable carriers and their formulations aredescribed in Remington's Pharmaceutical Sciences, 16th ed. 1980, MackPublishing Co. In addition, such compositions can contain flt3-Lcomplexed with polyethylene glycol (PEG), metal ions, or incorporatedinto polymeric compounds such as polyacetic acid, polyglycolic acid,hydrogels, etc., or incorporated into liposomes, microemulsions,micelles, unilamellar or multilamellar vesicles, erythrocyte ghosts orspheroblasts. Such compositions will influence the physical state,solubility, stability, rate of in vivo release, and rate of in vivoclearance of flt3-L. Flt3-L can also be conjugated to antibodies againsttissue-specific receptors, ligands or antigens, or coupled to ligands oftissue-specific receptors. Where the flt3 receptor is found onneoplastic cells, the flt3-L may be conjugated to a toxin whereby flt3-Lis used to deliver the toxin to the specific site, or may be used tosensitize such neoplastic cells to subsequently administeredanti-neoplastic agents.

Flt3-L can be administered topically, parenterally, or by inhalation.The term “parenteral” includes subcutaneous injections, intravenous,intramuscular, intracisternal injection, or infusion techniques. Thesecompositions will typically contain an effective amount of the flt3-L,alone or in combination with an effective amount of any other activematerial. Such dosages and desired drug concentrations contained in thecompositions may vary depending upon many factors, including theintended use, patient's body weight and age, and route ofadministration. Preliminary doses can be determined according to animaltests, and the scaling of dosages for human administration can beperformed according to art-accepted practices. Keeping the abovedescription in mind, typical dosages of flt3-L may range from about 10μg per square meter to about 1000 μg per square meter. A preferred doserange is on the order of about 100 μg per square meter to about 300 μgper square meter.

In addition to the above, the following examples are provided toillustrate particular embodiments and not to limit the scope of theinvention.

EXAMPLE 1 Preparation of Flt3-Receptor:Fc Fusion Protein

This example describes the cloning of murine flt3 cDNA, and theconstruction of an expression vector encoding a soluble murineflt3-receptor:Fc fusion protein for use in detecting cDNA clonesencoding flt3-L. Polymerase chain reaction (PCR) cloning of the flt3cDNA from a murine T-cell was accomplished using the oligonucleotideprimers and the methods as described by Lyman et al., Oncogene,8:815–822, (1993), incorporated herein by reference. The cDNA sequenceand encoded amino acid sequence for mouse flt3 receptor is presented byRosnet et el., Oncogene, 6:1641–1650, (1991), hereby incorporated byreference. The mouse flt3 protein has a 542 amino acid extracellulardomain, a 21 amino acid transmembrane domain, and a 437 amino acidcytoplasmic domain.

Prior to fusing the murine flt3 cDNA to the N-terminus of cDNA encodingthe Fc portion of a human IgG1 molecule, the amplified mouse flt3 cDNAfragment was inserted into Asp718-NotI site of pCAV/NOT, described inPCT Application WO 90/05183. DNA encoding a single chain polypeptidecomprising the Fc region of a human IgG1 antibody was cloned into theSpeI site of the pBLUESCRIPT SK® vector, which is commercially availablefrom Stratagene Cloning Systems, La Jolla, Calif. This plasmid vector isreplicable in E. coli and contains a polylinker segment that includes 21unique restriction sites. A unique BglII site was introduced near the 5′end of the inserted Fc encoding sequence, such that the BglII siteencompasses the codons for amino acids three and four of the Fcpolypeptide.

The encoded Fc polypeptide extends from the N-terminal hinge region tothe native C-terminus, i.e., is an essentially full-length antibody Fcregion. Fragments of Fc regions, e.g., those that are truncated at theC-terminal end, also may be employed. The fragments preferably containmultiple cysteine residues (at least the cysteine residues in the hingereaction) to permit interchain disulfide bonds to form between the Fcpolypeptide portions of two separate flt3:Fc fusion proteins, formingdimers as discussed above.

An Asp718 restriction endonuclease cleavage site was introduced upstreamof the flt3 coding region. An Asp 718-NotI fragment of mouse flt3 cDNA(comprising the entire extracellular domain, the transmembrane region,and a small portion of the cytoplasmic domain) was isolated. Theabove-described Asp718-NotI flt3 partial cDNA was cloned into thepBLUESCRIPT SK® vector containing the Fc cDNA, such that the flt3 cDNAis positioned upstream of the Fc cDNA. Single stranded DNA derived fromthe resulting gene fusion was mutagenized by the method described inKunkel (Proc. Natl. Acad. Sci. USA 82:488, 1985) and Kunkel et al.(Methods in Enzymol. 154:367, 1987) in order to perfectly fuse theentire extracellular domain of flt3 to the Fc sequence. The mutagenizedDNA was sequenced to confirm that the proper nucleotides had beenremoved (i.e., transmembrane region and partial cytoplasmic domain DNAwas deleted) and that the flt3 and Fc sequences were in the same readingframe. The fusion cDNA was then excised and inserted into a mammalianexpression vector designated sfHAV-EO 409 which was cut with SalI-NotI,and the SalI and Asp718 ends blunted. The sfHAV-EO vector (also known aspDC406) is described by McMahan et al. (EMBO J., 10; No. 10: 2821–2832(1991)).

Flt3:Fc fusion proteins preferably are synthesized in recombinantmammalian cell culture. The flt3:Fc fusion-containing expression vectorwas transfected into CV-1 cells (ATCC CCL 70) and COS-7 cells (ATCC CRL1651), both derived from monkey kidney. Flt3:Fc expression level wasrelatively low in both CV-1 and COS-7 cells. Thus, expression in 293cells (transformed primary human embryonal kidney cells, ATCC CRL 1573)was attempted.

The 293 cells transfected with the sfHAV-EO/flt3:Fc vector werecultivated in roller bottles to allow transient expression of the fusionprotein, which is secreted into the culture medium via the flt3 signalpeptide. The fusion protein was purified on protein A Sepharose columns,eluted, and used to screen cells for the ability to bind flt3:Fc, asdescribed in Examples 2 and 3.

EXAMPLE 2 Screening Cells for Flt3:Fc Binding

Approximately 100 different primary cells and cell lines falling intothe following general categories: primary murine fetal brain cells,murine fetal liver cell lines, rat fetal brain cell lines, human lungcarcinoma (fibroblastoid) cell lines, human and murine lymphoid andmyeloid cell lines were assayed for flt3:Fc binding. Cell lines wereincubated with flt3:Fc, followed by a biotinylated anti-human Fcantibody, followed by streptavidin-phycoerythrin (Becton Dickinson). Thebiotinylated antibody was purchased from Jackson ImmunoresearchLaboratories. Streptavidin binds to the biotin molecule attached to theanti-human Fc antibody, which in turn binds to the Fc portion of theflt3:Fc fusion protein. Phycoerythrin is a fluorescent phycobiliproteinwhich serves as a detectable label. The level of fluorescence signal wasmeasured for each cell type using a FACScan® flow cytometer (BectonDickinson). The cell types deemed positive for flt3:Fc binding wereidentified.

EXAMPLE 3 Isolation and Cloning of Flt3 L cDNA from Murine T-Cell cDNALibrary

A murine T-cell cDNA library of cell line P7B-0.3A4 was chosen as apossible source of flt3-L cDNA. P7B-0.3A4 is a murine T cell clone thatis Thy1.2⁺, CD4⁻, CD8⁻, TCRab^(±), CD44⁺. It was originally cloned at acell density of 0.33 cells/well in the presence of rHuIL-7 andimmobilized anti-CD3 MAb, and was grown in continuous culture for morethan 1 year by passage once a week in medium containing 15 ng/mlrHuIL-7. The parent cell line was derived from lymph node cells of SJL/Jmice immunized with 50 nmoles PLP₁₃₉₋₁₅₁ peptide and 100 μgMycobacterium tuberculosis H37Ra in Incomplete Freund's Adjuvant. PLP isthe proteolipid protein component of the myelin sheath of the centralnervous system. The peptide composed of amino acids 139–151 haspreviously been shown to be the encephalogenic peptide in experimentalautoimmune encephalomyelitis (EAE), a murine model for multiplesclerosis in SJL/J mice. (Touhy, V. K., Z. Lu, R. A. Sobel, R. A.Laursen and M. B. Lees; 1989. Identification of an encephalitogenicdeterminant of myelin proteolipid protein for SJL mice. J. Immunol.142:1523.) After the initial culture in the presence of antigen, theparent cell line, designated PLP7, had been in continuous culture withrHuIL-7 (and without antigen) for more than 6 months prior to cloning.

P7B-0.3A4 proliferates only in response to very high concentrations ofPLP₁₃₉₋₁₅₁ peptide in the presence of irradiated syngeneic splenocytesand is not encephalogenic or alloresponsive. This clone proliferates inresponse to immobilized anti-CD3 MAb, IL-2, and IL-7, but not IL-4.

Binding of flt3:Fc was observed on murine T-cells and human T-cells, andtherefore a murine T-cell line was chosen (0.3A4) due to its ease ofgrowth. A murine 0.3A4 cDNA library in sfHAV-EO was prepared asdescribed in McMahan et al. (EMBO J., 10; No:10; 2821–2832 1991).sfHAV-EO is a mammalian expression vector that also replicates in E.coli. sfHAV-EO contains origins of replication derived from SV40,Epstein-Barr virus and pBR322 and is a derivative of HAV-EO described byDower et al., J Immunol. 142:4314 (1989). sfHAV-EO differs from HAV-EOby the deletion of the intron present in the adenovirus 2 tripartiteleader sequence in HAV-EO. Briefly, murine T-cell cDNA was cloned intothe SalI site of sfHAV-EO by an adaptor method similar to that describedby Haymerle et al (Nucl. Acids Res. 14:8615, 1986), using the followingoligonucleotide adapter pair:

5′ TCGACTGGAACGAGACGACCTGCT 3′ SEQ ID NO:3 3′     GACCTTGCTCTGCTGGACGA5′ SEQ ID NO:4Double-stranded, blunt-ended, random-primed cDNA was prepared from 0.3A4poly (A)+ RNA essentially as described by Gubler and Hoffman, Gene,25:263–269 (1983), using a Pharmacia DNA kit. The above adapters wereadded to the cDNA as described by Haymerle et al. Low molecular weightmaterial was removed by passage over Sephacryl S-1000 at 65° C., and thecDNA was ligated into sfHAV-EO410, which had previously been cut withSalI and ligated to the same oligonucleotide pair. This vector isdesignated as sfHAV-EO410. DNA was electroporated (Dower et al., NucleicAcids Res., 16:6127–6145, (1988) into E. coli DH10B, and after one hourgrowth at 37° C., the transformed cells were frozen in one milliliteraliquots in SOC medium (Hanahan et al., J. Mol. Biol., 166:557–580,(1983) containing 20% glycerol. One aliquot was titered to determine thenumber of ampcillin-resistant colonies. The resulting 0.3A4 library had1.84 million clones.

E. coli strain DH10B cells transfected with the cDNA library insfHAV-EO410 were plated to provide approximately 1600 colonies perplate. Colonies were scraped from each plate, pooled, and plasmid DNAprepared from each pool. The pooled DNA, representing about 1600colonies, was then used to transfect a sub-confluent layer ofCV-1/EBNA-1 cells using DEAE-dextran followed by chloroquine treatment,similar to that described by Luthman et al., Nucl. Acids Res. 11:1295,(1983) and McCutchan et al., J. Natl. Cancer Inst. 41:351, (1986). TheCV-1/EBNA-1 cell line (ATCC CRL10478) constitutively expresses EBVnuclear antigen-1 driven from the CMV immediate-early enhancer/promoter.CV1-EBNA-1 was derived from the African Green Monkey kidney cell lineCV-1 (ATCC CCL 70), as described by McMahan et al. (EMBO J. 10:2821,1991).

In order to transfect the CV-1/EBNA-1 cells with the cDNA library, thecells were maintained in complete medium (Dulbecco's modified Eagle'smedia (DMEM) containing 10% (v/v) fetal calf serum (FCS), 50 U/mlpenicillin, 50 U/ml streptomycin, 2 mM L-glutamine) and were plated at adensity of about 2×10⁵ cells/well on single-well chambered slides(Lab-Tek). Slides were pretreated with 1 ml human fibronectin (10 μg/mlin PBS) for 30 minutes followed by 1 wash with PBS. Media was removedfrom the adherent cell layer and replaced with 1.5 ml complete mediumcontaining 66.6 μM chloroquine sulfate. Two-tenths ml of DNA solution (2μg DNA, 0.5 mg/ml DEAE-dextran in complete medium containingchloroquine) was then added to the cells and incubated for 5 hours.Following the incubation, the media was removed and the cells shocked byaddition of complete medium containing 10% DMSO for 2.5 to 20 minutesfollowed by replacement of the solution with fresh complete medium. Thecells were cultured for 2 to 3 days to permit transient expression ofthe inserted sequences.

Transfected monolayers of CV-1/EBNA-1 cells were assayed for expressionof flt3-L by slide autoradiography essentially as described by Gearinget al. (EMBO J. 8:3667, 1989). Transfected CV-1/EBNA-1 cells (adhered tochambered slides) were washed once with binding medium with nonfat drymilk (BM-NFDM) (RPMI medium 1640 containing 25 mg/ml bovine serumalbumin (BSA), 2 mg/ml sodium azide, 20 mM HEPES, pH 7.2, and 50 mg/mlnonfat dry milk). Cells were then incubated with flt3:Fc in BM-NFDM (1μg/ml) for 1 hour at room temperature. After incubation, the cellmonolayers in the chambered slides were washed three times with BM-NFDMto remove unbound flt3:Fc fusion protein and then incubated with 40ng/ml ¹²⁵I-mouse anti-human Fc antibody (described below) (a 1:50dilution) for 1 hour at room temperature. The cells were washed threetimes with BM-NFDM, followed by 2 washes with phosphate-buffered saline(PBS) to remove unbound ¹²⁵I-mouse anti-human Fc antibody. The cellswere fixed by incubating for 30 minutes at room temperature in 2.5%glutaraldehyde in PBS, pH 7.3, washed twice in PBS and air dried. Thechamber slides containing the cells were exposed on a Phophorimager(Molecular Dynamics) overnight, then dipped in Kodak GTNB-2 photographicemulsion (6× dilution in water) and exposed in the dark for 3–5 days at4° C. in a light proof box. The slides were then developed forapproximately 4 minutes in Kodak D19 developer (40 g/500 ml water),rinsed in water and fixed in Agfa G433C fixer. The slides wereindividually examined with a microscope at 25–40× magnification andpositive cells expressing flt3-L were identified by the presence ofautoradiographic silver grains against a light background.

The mouse anti-human Fc antibody was obtained from Jackson Laboratories.This antibody showed minimal binding to Fc proteins bound to the Fcγreceptor. The antibody was labeled using the Chloramine T method.Briefly, a Sephadex G-25 column was prepared according to themanufacturer's instructions. The column was pretreated with 10 columnvolumes of PBS containing 1% bovine serum albumin to reduce nonspecificadsorption of antibody to the column and resin. Nonbound bovine serumalbumin was then washed from the column with 5 volumes of PBS lackingbovine serum albumin. In a microfuge tube 10 μg of antibody (dissolvedin 10 μl of PBS) was added to 50 μl of 50 mM sodium phosphate buffer (pH7.2) 2.0 mCi of carrier-free Na¹²⁵I was added and the solution was mixedwell. 15 μl of a freshly prepared solution of chloramine-T (2 mg/ml in0.1 M sodium phosphate buffer (pH 7.2)) was then added and the mixturewas incubated for 30 minutes at room temperature, and the mixture thenwas immediately applied to the column of Sephadex G-25. Theradiolabelled antibody was then eluted from the column by collecting100–150 μl fractions of eluate. Bovine serum albumin was added to theeluted fractions containing the radiolabeled antibody to a finalconcentration of 1%. Radioiodination yielded specific activities in therange of 5–10×10¹⁵ cpm/nmol protein.

Using the slide autoradiography approach, the approximately 1,840,000cDNAs were screened in pools of approximately 1,600 cDNAs until assay ofone transfectant pool showed multiple cells clearly positive for flt3:Fcbinding. This pool was then partitioned into pools of 500 and againscreened by slide autoradiography and a positive pool was identified.This pool was partitioned into pools of 100 and again screened.Individual colonies from this pool of 100 were screened until a clone(clone #6C) was identified which directed synthesis of a surface proteinwith detectable flt3:Fc binding activity. This clone was isolated, andits 0.88 kb cDNA insert was sequenced.

The nucleotide and encoded amino acid sequences of the coding region ofthe murine flt3-ligand cDNA of clone #6C are presented in SEQ ID NOs:1and 2. The cDNA insert is 0.88 kb in length. The open-reading framewithin this sequence could encode a protein of 231 amino acids. Thus,DNA and encoded amino acid sequences for the 231-amino acid open readingframe are presented in SEQ ID NOs:1 and 2. The protein of SEQ ID NO:2 isa type I transmembrane protein, with an N-terminal signal peptide (aminoacids 1 to 27), an extracellular domain (amino acids 28–188) atransmembrane domain (amino acids 189–211) and a cytoplasmic domain(amino acids 212–231). The predicted molecular weight of the nativeprotein following cleavage of the signal sequence is 23,164 daltons. Themature protein has an estimated pI of 9.372. There are 56 bp of 5′noncoding sequence and 126 bp of 3′ non-coding sequence flanking thecoding region, including the added cDNA adapters. The above-describedcloning procedure also produced a murine flt3 ligand clone #5H, which isidentical to the #6C clone beginning at nucleotide 49 and continuingthrough nucleotide 545 (corresponding to amino acid 163) of SEQ ID NO:1.The #5H clone completely differs from that point onward, and representsan alternate splicing construct.

The vector of sfHAV-EO410 containing the flt3-L cDNA of clone #6C in E.coil DH10B cells was deposited with the American Type CultureCollection, Rockville, Md., USA (ATCC) on Apr. 20, 1993, and assignedaccession number ATCC 69286. The deposit was made under the terms of theBudapest Treaty.

EXAMPLE 4 Cloning of cDNA Encoding Human Flt3-L

A cDNA encoding human flt3-L was cloned from a human clone 22 T cellλgt10 random primed cDNA library as described by Sims et al., PNAS,86:8946–8950 (1989). The library was screened with a 413 bp Ple Ifragment corresponding to the extracellular domain of the murine flt3-L(nucleotides 103–516 of SEQ ID NO:1). The fragment was random primed,hybridized overnight to the library filters at 55° C. in oligoprehybridization buffer. The fragment was then washed at 55° C. at2×SSC/0.1% SDS for one hour, followed by 1×SSC/0.1% SDS for one hour andthen by 0.5×SSC/0,1% SDS for one hour. The DNA from the positive phageplaques was extracted, and the inserts were amplified by PCR usingoligonucleotides specific for the phage arms. The DNA then wassequenced, and the sequence for clone #9 is shown in SEQ ID NO:5.Additional human flt3-L cDNA was isolated from the same λgt10 randomprimed cDNA library as described above by screening the library with afragment of the extracellular domain of the murine clone #5H cDNAcomprising a cDNA sequence essentially corresponding to nucleotides128–541 of SEQ ID NO:1.

Sequencing of the 988 bp cDNA clone #9 revealed an open reading frame of705 bp surrounded by 29 bp of 5′ non-coding sequence and 250 bp of 3′non-coding sequence. The 3′ non-coding region did not contain a poly-Atail. There were no in-frame stop codons upstream of the initiatormethionine. The open reading frame encodes a type I transmembraneprotein of 235 amino acids as shown by amino acids 1–235 of SEQ ID NO:6.The protein has an N-terminal signal peptide of alternatively 26 or 27amino acids. There exists a slightly greater probability that theN-terminal signal peptide is 26 amino acids in length than 27 aminoacids in length. The signal peptide is followed by a 156 or a 155 aminoacid extracellular domain (for signal peptides of 26 and 27 amino acids,respectively); a 23 amino acid transmembrane domain and a 30 amino acidcytoplasmic domain. Human flt3-L shares overall 72% amino acid identityand 78% amino acid similarity with murine flt3-L. The vector pBLUESCRIPTSK(−) containing the human flt3-L cDNA of clone #9 was deposited withthe American Type Culture Collection, Rockville, Md., USA (ATCC) on Aug.6, 1993 and assigned acession number ATCC 69382. The deposit was madeunder the terms of the Budapest Treaty.

EXAMPLE 5 Expression of Flt3-L in Yeast

For expression of soluble flt3-L in yeast, synthetic oligonucleotideprimers were used to amplify via PCR (Mullis and Faloona, Meth. Enzymol.155:335–350, 1987) the entire extracellular coding domain of flt3-Lbetween the end of the signal peptide and the start of the transmembranesegment. The 5′ primer(5′-AATTGGTACCTTGGATAAAAGAGACTACAAGGACGACGATGACAAGACACCTGACTGTTACTTCAGCCAC-3′)SEQ ID NO:7 encoded a portion of of the alpha factor leader and anantigenic octapeptide, the FLAG sequence fused in-frame with thepredicted mature N-terminus of flt3-L. The 3′ oligonucleotide(5′-ATATGGATCCCTACTGCCTGGGCCGAGGCTCTGGGAG-3′) SEQ ID NO:8 created atermination codon following Gln-189, just at the putative transmembraneregion. The PCR-generated DNA fragment was ligated into a yeastexpression vector (for expression in K. lactis) that directs secretionof the recombinant product into the yeast medium (Fleer et. al., Gene,107:285–195 (1991); and van den Berg et. al., Bio/Technology, 8:135–139(1990)). The FLAG:flt3-L fusion protein was purified from yeast broth byaffinity chromatography as previously described (Hopp et. al.,Biotechnology, 6:1204–1210, 1988).

EXAMPLE 6 Monoclonal Antibodies to Flt3-L

This example illustrates a method for preparing monoclonal antibodies toflt3-L. Flt3-L is expressed in mammalian host cells such as COS-7 orCV-1/EBNA-1 cells and purified using flt3:Fc affinity chromatography.Purified flt3-L, a fragment thereof such as the extracellular domain,synthetic peptides or cells that express flt3-L can be used to generatemonoclonal antibodies against flt3-L using conventional techniques, forexample, those techniques described in U.S. Pat. No. 4,411,993. Briefly,mice are immunized with flt3-L as an immunogen emulsified in completeFreund's adjuvant, and injected in amounts ranging from 10–100 μgsubcutaneously or intraperitoneally. Ten to twelve days later, theimmunized animals are boosted with additional flt3-L emulsified inincomplete Freund's adjuvant. Mice are periodically boosted thereafteron a weekly to bi-weekly immunization schedule. Serum samples areperiodically taken by retro-orbital bleeding or tail-tip excision totest for flt3-L antibodies by dot blot assay, ELISA (Enzyme-LinkedImmunosorbent Assay) or inhibition of flt3 binding.

Following detection of an appropriate antibody titer, positive animalsare provided one last intravenous injection of flt3-L in saline. Threeto four days later, the animals are sacrificed, spleen cells harvested,and spleen cells are fused to a murine myeloma cell line, e.g., NS1 orpreferably P3x63Ag8.653 (ATCC CRL 1580). Fusions generate hybridomacells, which are plated in multiple microtiter plates in a HAT(hypoxanthine, aminopterin and thymidine) selective medium to inhibitproliferation of non-fused cells, myeloma hybrids, and spleen cellhybrids.

The hybridoma cells are screened by ELISA for reactivity againstpurified flt3-L by adaptations of the techniques disclosed in Engvall etal., Immunochem. 8:871, 1971 and in U.S. Pat. No. 4,703,004. A preferredscreening technique is the antibody capture technique described inBeckmann et al., (J. Immunol. 144:4212, 1990) Positive hybridoma cellscan be injected intraperitoneally into syngeneic BALB/c mice to produceascites containing high concentrations of anti-flt3-L-L monoclonalantibodies. Alternatively, hybridoma cells can be grown in vitro inflasks or roller bottles by various techniques. Monoclonal antibodiesproduced in mouse ascites can be purified by ammonium sulfateprecipitation, followed by gel exclusion chromatography. Alternatively,affinity chromatography based upon binding of antibody to protein A orprotein G can also be used, as can affinity chromatography based uponbinding to flt3-L.

EXAMPLE 7 Use of Flt3-L Alone and in Combination with IL-7 or IL-3

This example demonstrates the stimulation and proliferation of AA4.1⁺fetal liver cells by compositions containing flt3-L and IL-7; as well asthe stimulation and proliferation of c-kit-positive (c-kit⁺) cells bycompositions containing flt3-L and IL-3.

AA4.1-positive (AA4.1⁺) expressing cells were isolated from the liversof day 14 fetal C57BL/6 mice by cell panning in Optilux 100 mm plasticPetri dishes (Falcon No. 1001, Oxnard, Calif.). Plates were coatedovernight at 4° C. in PBS plus 0.1% fetal bovine serum (FBS) containing10 μg/ml AA4.1 antibody (McKearn et. al., J. Immunol., 132:332–339,1984) and then washed extensively with PBS plus 1% FBS prior to use. Asingle cell suspension of liver cells was added at 10⁷ cells/dish in PBSplus 1% FBS and allowed to adhere to the plates for two hours at 4° C.The plates were then extensively washed, and the adhering cells wereharvested by scraping for analysis or further use in the hematopoiesisassays described below. FACS analysis using AA4.1 antibody demonstrateda >95% AA4.1⁺ cell population.

C-kit⁺ pluripotent stem cells were purified from adult mouse bone marrow(de Vries et. al. , J. Exp. Med., 176:1503–1509, 1992; and Visser and deVries, Methods in Cell Biol., 1993, submitted). Low density cells(≦1.078 g/cm³) positive for the lectin wheat germ agglutinin andnegative for the antigens recognized by the B220 and 15–1.4.1 (Visseret. al., Meth. in Cell Biol., 33:451–468, 1990) monoclonal antibodies,could be divided into sub-populations of cells that do and do notexpress c-kit by using biotinylated Steel factor. The c-kit⁺fraction hasbeen shown to contain pluripotent hematopoietic stem cells (de Vries et.al., Science 255:989–991, 1992; Visser and de Vries, Methods in CellBiol., 1993, submitted; and Ware et. al., 1993, submitted).

AA4.1+ Fetal liver cells were cultured in recombinant IL-7 (U.S. Pat.No. 4,965,195) at 100 ng/ml and recombinant flt3-L at 250 ng/ml. Flt3-Lwas used in three different forms in the experiments: (1) as present onfixed, flt3-L-transfected CV1/EBNA cells; (2) as concentrated culturesupernatants from these same flt3-L-transfected CV1/EBNA cells; and (3)as a purified and isolated polypeptide preparation from yeastsupernatant as described in Example 5.

Hematopoiesis Assavs

The proliferation of c-kit⁺ stem cells, fetal liver AA4.1⁺ cells wasassayed in [3H]-thymidine incorporation assays as essentially describedby devries et. al., J. Exp. Med., 173:1205–1211, 1991. Purified c-kit⁺stem cells were cultured at 37° C. in a fully humidified atmosphere of6.5% CO₂ and 7% O₂ in air for 96 hours. Murine recombinant IL-3 was usedat a final concentration of 100 ng/ml. Subsequently, the cells werepulsed with 2 μCi per well of [³H]-thymidine (81 Ci/mmol; AmershamCorp., Arlington Heights, Ill.) and incubated for an additional 24hours. AA4.1⁺ cells (approximately 20,000 cells/well) were incubated inIL-7, flt3-L and flt3-L +IL-7 for 48 hours, followed by [³H]-thymidinepulse of six hours. The results of flt3-L and IL-7 are shown in Table I,and results of flt3-L and IL-3 are shown in Table II.

TABLE I Effect of Flt3-L and IL-7 on Proliferation of AA4.1+ Fetal LiverCells. Factor Control flt3-L IL-7 flt3-L + IL-7 [³H]-thymidine 100 1000100 4200 incorporation (CPM)

The combination of flt3-L and IL-7 produced a response that wasapproximately four-fold greater than flt3-L alone and approximately40-fold greater than IL-7 alone.

TABLE II Effect of Flt3-L and IL-3 on Proliferation of C-kit+ Cells.Factor Control (vector alone) flt3-L IL-3 flt3-L + IL-3 [³H]-thymidine100 1800 3000 9100 incorporation (CPM)

Culture supernatant from CV1/EBNA cells transfected with flt3-L cDNAstimulated the proliferation of c-kit⁺ stem cells approximately 18-foldgreater than the culture supernatant of CV1/EBNA cells transfected withthe expression vector alone. Addition of IL-3 to flt3-L containingsupernatant showed a synergistic effect, with approximately twice thedegree of proliferation observed than would be expected if the effectswere additive.

EXAMPLE 8 Construction of FIt3-L:Fc Fusion Protein

This example describes a methof for constructing a fusion proteincomprising an extracellular region of the flt3-L and the Fc domain of ahuman immunoglobulin. The methods are essentially the same as thosedescribed in Example 1 for construction of a flt3:Fc fusion protein.

Prior to fusing a flt3-L cDNA to the N-terminus of cDNA encoding the Fcportion of a human IgG1 molecule, the flt3-L cDNA fragment is insertedinto Asp718-NotI site of pCAV/NOT, described in PCT Application WO90/05183. DNA encoding a single chain polypeptide comprising the Fcregion of a human IgG1 antibody is cloned into the SpeI site of thepBLUESCRIPT SK® vector, which is commercially available from StratageneCloning Systems, La Jolla, Calif. This plasmid vector is replicable inE. coli and contains a polylinker segment that includes 21 uniquerestriction sites. A unique BglII site is then introduced near the 5′end of the inserted Fc encoding sequence, such that the BglII siteencompasses the codons for amino acids three and four of the Fc polypeptide.

The encoded Fc polypeptide extends from the N-terminal hinge region tothe native C-terminus, i.e., is an essentially full-length antibody Fcregion. Fragments of Fc regions, e.g., those that are truncated at theC-terminal end, also may be employed. The fragments preferably containmultiple cysteine residues (at least the cysteine residues in the hingereaction) to permit interchain disulfide bonds to form between the Fcpolypeptide portions of two separate flt3-L:Fc fusion proteins, formingdimers.

An Asp718-Stul partial cDNA of fit3-L in pCAV/NOT can be cloned into aAsp718-SpeI site of pBLUESCRIPT SK® vector containing the Fc cDNA, suchthat the fit3-L cDNA is positioned upstream of the Fc cDNA. The sequenceof single stranded DNA derived from the resulting gene fusion can beaffected by template-directed mutagensis described by Kunkel (Proc.Natl. Acad. Sci. USA 82:488, 1985) and Kunkel et al. (Methods inEnzymol. 154:367, 1987) in order to perfectly fuse the entireextracellular domain of flt3-L to the Fc sequence. The resulting DNA canthen be sequenced to confirm that the proper nucleotides are removed(i.e., transmembrane region and partial cytoplasmic domain DNA aredeleted) and that flt3-L and Fc sequences are in the same reading frame.The fusion cDNA is then excised and inserted using conventional methodsinto the mammalian expression vector pCAV/NOT which is cut with Asp718-NotI.

Flt3-L:Fc fusion proteins preferably are synthesized in recombinantmammalian cell culture. The flt3-L:Fc fusion-containing expressionvector is then transfected into CV-1 cells (ATCC CCL 70) or COS-7 cells(ATCC CRL 1651). Expression in 293 cells (transformed primary humanembryonal kidney cells, ATCC CRL 1573) also is feasible.

The 293 cells transfected with the pCAV/NOT/flt3-L:Fc vector arecultivated in roller bottles to allow transient expression of the fusionprotein, which is secreted into the culture medium via the flt3-L signalpeptide. The fusion protein can be purified on protein A Sepharosecolumns.

EXAMPLE 9 Generation of Transgenic Mice That Overexpress Flt3-L

This example describes a procedure used to generate transgenic mice thatoverexpress flt3-L. Flt3-L-overexpressing transgenic mice were studiedto determine the biological effects of overexpression. Mouse (B16/J)pronuclei were microinjected with flt3-L DNA according to the methoddescribed by Gordon et al., Science 214:1244–1246, (1981). In general,fertilized mouse eggs having visible pronuclei were first placed on aninjection chamber and held in place with a small pipet. An injectionpipet was then used to inject the gene encoding the flt3-L (clone #6C)into the pronuclei of the egg. Injected eggs were then either (i)transferred into the oviduct of a 0.5 day p.c. pseudopregnant female;(ii) cultured in vitro to the two-cell stage (overnight) and transferredinto the oviduct of a 0.5 day p.c. pseudopregnant female; or (iii)cultured in vitro to the blastocyst stage and transferred into theuterus of a 2.5 day p.c. pseudopregnant female. Preferably, either ofthe first two options can be used since they avoid extended in vitroculture, and preferably, approximately 20–30 microinjected eggs shouldbe transferred to avoid small litters.

EXAMPLE 10 Flt3-L Stimulates Proliferation of Erythroid Cells in theSpleen

This example describes the effect of flt3-L on the production oferythroid cells in the spleen of transgenic mice. Transgenic mice weregenerated according to the procedures of Example 10. The mice weresacrificed and each intact spleen was made into a single cellsuspension. The suspended cells were spun and then resuspended in 10 mlof medium that contained PBS+1% fetal bovine serum. Cell counts wereperformed thereon using a hemocytometer. Each cell specimen was countedwith Trypan Blue stain to obtain a total viable cell count permilliliter of medium according to the following formula: (RBC+WBC)/ml,wherein RBC is the red blood cell count and WBC means the white bloodcell count. Each specimen then was counted with Turk's stain to obtain atotal white blood cell count per milliliter of medium. The total redblood cell count per milliliter was calculated for each specimen bysubtracting the total white blood cell count per milliliter from thetotal viable cell count per milliliter. The results are shown in thefollowing Table III.

TABLE III Erythroid Cell Proliferation in Flt3-L-OverexpressingTransgenic Mice Spleen Total Viable Cell Total White Cell Total RedBlood Cell Mouse (million cells/ml) (million cells/ml) (millioncells/ml) Control 1 29.7 27 2.7 Control 2 31 24.6 6.4 Transgenic 1 44.725.6 19.1 Transgenic 2 37.3 28.4 8.9

From the data of Table III, the white blood cell counts per milliliterwere approximately the same as the control mice. However, the red bloodcell counts from the spleens of the two transgenic mice wereapproximately two to three-fold greater than observed in the controlmice. Flt3-L stimulates an increase in cells of the erythroid lineage,possibly through stimulation of erythroid proogenitor cells, through thestimulation of cells that produce erythropoietin, or by blocking amechanism that inhibits erythropoiesis.

EXAMPLE 11 Flt3-L Stimulates Proliferation of T Cells and Early B Cells

Bone marrow from 9 week old transgenic mice generated according toExample 10 was screened for the presence of various T and B cellphenotype markers using antibodies that are immunoreactive with suchmarkers. The following markers were investigated: the B220 marker, whichis specific to the B cell lineage; surface IgM marker (sIgM), which isspecific to mature B cells; the S7 (CD43) marker, which is an early Bcell marker; the Stem Cell Antigen-1 (SCA-1) marker, which is a markerof activated T cells and B cells; CD4, which is a marker for helper Tcells and some stem cells; and the Mac-1 marker, which is specific tomacrophages, were screened using well known antibodies against suchmarkers. The following Table IV shows the data obtained from screeningthe bone marrow. Two transgenic mice from the same litter were analyzedagainst a normal mouse from the same litter (control), and an unrelatednormal mouse (control).

TABLE IV Effect of flt3-L Overexpression in Transgenic Mice Percentageof Positive Cells Unrelated Littermate Marker Control Control Transgenic#1 Transgenic #2 B220 30.64 27.17 45.84 48.78 sIgM 3.54 2.41 1.94 1.14S7(CD43) 54.43 45.44 46.11 50.59 SCA-1 10.92 11.74 19.45 27.37 CD4 6.948.72 12.21 14.05 Mac-1 36.80 27.15 21.39 18.63

The above data indicate that flt3-L overexpression in mice leads to anincrease in the number of B cells, as indicated by the increase B220⁺cells and SCA-1⁺ cells. Analysis of B220⁺ cells by FACS indicated anincrease in proB cells (HSA⁻, S7⁺). The increase in CD4⁺ cells indicatedan approximate two-fold increase in T cells and stem cells. The decreasein cells having the sIgM marker indicated that flt3-L does not stimulateproliferation of mature B cells. These data indicate that flt3-Lincreases cells with a stem cell, T cell or an early B cell phenotype,and does not stimulate proliferation of mature B cells or macrophages.

EXAMPLE 12 Analysis of the Thymus from Flt3-L-Over-expressing Mice

This Example describes the analysis of the thymus from the transgenicmice generated according to the procedure of Example 10. Six adult mice,each approximately three months of age, were sacrificed. The thymus fromeach mouse was removed and a single cell suspension was made.

FACS analysis demonstrated that no total change in cell number occurredand that the mice showed no change in the ratios of maturing thymocytesusing the markers: CD4 vs. CD8; CD3 vs. αβTCR (T cell receptor); and CD3vs. γδTCR (T cell receptor). However, a change in the ratios of certaincell types within the CD4⁻ and CD8⁻ compartment (i.e., the earliestcells with respect to development; which represent approximately 2% to3% of total thymus cells) occurred. Specifically, CD4⁻ and CD8⁻ cells inthe thymus develop in three stages. Stage 1 represents cells having thePgp-1⁺⁺, HSA⁺ and IL-2 receptor-negative (“IL-2R-”) markers. After stage1, thymic cells develop to stage 2 consisting of cells having Pgp-1⁺,HSA⁺⁺, and IL-2R⁺⁺ markers, and then to stage 3, characterized by cellshaving Pgp-1^(+/−), HSA⁺⁺, and IL-2R⁻ markers. Thymic cells in stage 2of the transgenic mice were reduced by about 50%, while the populationof cells in stage 3 was proportionately increased. These data suggestthat flt3-L drives the thymic cells from stage 2 to stage 3 ofdevelopment, indicating that flt3-L is active on early T cells.

EXAMPLE 13 Use of Flt3-L in Peripheral Stem Cell Transplantation

This Example describes a method for using flt3-L in autologousperipheral stem cell (PSC) or peripheral blood progenitor cell (PBPC)transplantation. Typically, PBPC and PSC transplantation is performed onpatients whose bone marrow is unsuitable for collection due to, forexample, marrow abnormality or malignant involvement.

Prior to cell collection, it may be desirable to mobilize or increasethe numbers of circulating PBPC and PSC. Mobilization can improve PBPCand PSC collection, and is achievable through the intravenousadministration of flt3-L to the patients prior to collection of suchcells. Other growth factors such as CSF-1, GM-CSF, SF, G-CSF, EPO, IL-1,IL-2, IL-3, IL-4, IL-5, IL-6, IL-7, IL-8, IL-9, IL-10, IL-11, IL-12,IL-13, IL-14, IL-15, GM-CSF/IL-3 fusion proteins, LIF, FGF andcombinations thereof, can be likewise administered in sequence, or inconcurrent combination with flt3-L. Mobilized or non-mobilized PBPC andPSC are collected using apheresis procedures known in the art. See, forexample, Bishop et al., Blood, vol. 83, No. 2, pp. 610–616 (1994).Briefly, PBPC and PSC are collected using conventional devices, forexample, a Haemonetics Model V50 apheresis device (Haemonetics,Braintree, Mass.). Four-hour collections are performed typically no morethan five times weekly until approximately 6.5×10⁸ mononuclear cells(MNC)/kg patient are collected. Aliquots of collected PBPC and PSC areassayed for granulocyte-macrophage colony-forming unit (CFU-GM) contentby diluting approximately 1:6 with Hank's balanced salt solution withoutcalcium or magnesium (HBSS) and layering over lymphocyte separationmedium (Organon Teknika, Durham, N.C.). Following centrifugation, MNC atthe interface are collected, washed and resuspended in HBSS. Onemilliliter aliquots containing approximately 300,000 MNC, modifiedMcCoy's 5A medium, 0.3% agar, 200 U/ml recombinant human GM-CSF, 200u/mL recombinant human IL-3, and 200 u/mL recombinant human G-CSF arecultured at 37° C. in 5% CO₂ in fully humidified air for 14 days.Optionally, flt3-L or GM-CSF/IL-3 fusion molecules (PIXY 321) may beadded to the cultures. These cultures are stained with Wright's stain,and CFU-GM colonies are scored using a dissecting microscope (Ward etal., Exp. Hematol., 16:358 (1988). Alternatively, CFU-GM colonies can beassayed using the CD34/CD33 flow cytometry method of Siena et al.,Blood, Vol. 77, No. 2, pp 400–409 (1991), or any other method known inthe art.

CFU-GM containing cultures are frozen in a controlled rate freezer(e.g., Cryo-Med, Mt. Clemens, Mich.), then stored in the vapor phase ofliquid nitrogen. Ten percent dimethylsulfoxide can be used as acryoprotectant. After all collections from the patient have been made,CFU-GM containing cultures are thawed and pooled. The thawed cellcollection either is reinfused intravenoulsy to the patient or expandedex vivo prior to reinfusion. Ex vivo expansion of pooled cells can beperformed using flt3-L as a growth factor either alone, sequentially orin concurrent combination with other cytokines listed above. Methods ofsuch ex vivo expansion are well known in the art. The cells, eitherexpanded or unexpanded, are reinfused intravenously to the patient. Tofacilitate engraftment of the transplanted cells, flt3-L is administeredsimultaneously with, or subsequent to, the reinfusion. Suchadministration of flt3-L is made alone, sequentially or in concurrentcombination with other cytokines selected from the list above.

EXAMPLE 14 Purification of Hematopoietic Progenitor and Stem Cells UsingFlt3-L

This Example describes a method for purifying hematopoietic progenitorcells and stem cells from a suspension containing a mixture of cells.Cells from bone marrow and peripheral blood are collected usingconventional procedures. The cells are suspended in standard media andthen centrifuged to remove red blood cells and neutrophils. Cellslocated at the interface between the two phases (also known in the artas the buffy coat) are withdrawn and resuspended. These cells arepredominantly mononuclear and represent a substantial portion of theearly hematopoietic progenitor and stem cells. The resulting cellsuspension then is incubated with biotinylated flt3-L for a sufficienttime to allow substantial flt3:flt3-L interaction. Typically, incubationtimes of at least one hour are sufficient. After incubation, the cellsuspension is passed, under the force of gravity, through a columnpacked with avidin-coated beads. Such columns are well known in the art,see Berenson, et al., J. Cell Biochem., 10D:239 (1986). The column iswashed with a PBS solution to remove unbound material. Target cells canbe released from the beads and from flt3-L using conventional methods.

1. An isolated polypeptide that binds to flt3, wherein said polypeptidecomprises amino acids 28–163 of SEQ ID NO:2.
 2. A pharmaceuticalcomposition comprising the polypeptide of claim 1 in an amountsufficient to stimulate proliferation of hematopoietic stem orprogenitor cells; and a pharmaceutically acceptable carrier, excipientor diluent.
 3. The isolated polypeptide of claim 1, wherein saidpolypeptide comprises amino acids 28–188 of SEQ ID NO:2.
 4. Apharmaceutical composition comprising the polypeptide of claim 3 in anamount sufficient to stimulate proliferation of hematopoietic stem orprogenitor cells; and a pharmaceutically acceptable carrier, excipientor diluent. pharmaceutically.
 5. The isolated polypeptide of claim 3,wherein said polypeptide comprises amino acids 1–231 of SEQ ID NO:2. 6.A pharmaceutical composition comprising the polypeptide of claim 5 in anamount sufficient to stimulate proliferation of hematopoietic stem orprogenitor cells; and a pharmaceutically acceptable carrier, excipientor diluent.
 7. The isolated polypeptide of claim 1, wherein saidpolypeptide comprises amino acids 1–188 of SEQ ID NO:2.
 8. Apharmaceutical composition comprising the polypeptide of claim 7 in anamount sufficient to stimulate proliferation of hematopoietic stem orprogenitor cells; and a pharmaceutically acceptable carrier, excipientor diluent.
 9. An isolated polypeptide that binds to flt3, wherein saidpolypeptide is encoded by encoded by vector sfHAV-EO410 in E. coil DH10Bcells having ATCC Accession No.
 69286. 10. A pharmaceutical compositioncomprising the polypeptide of claim 9 in an amount sufficient tostimulate proliferation of hematopoietic stem or progenitor cells; and apharmaceutically acceptable carrier, excipient or diluent.